First draft of the K2V specification
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# Specification of the Garage K2V API (K2V = Key/Key/Value)
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- We are storing triplets of the form `(partition key, sort key, value)` -> no
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user-defined fields, the client is responsible of writing whatever he wants
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in the value (typically an encrypted blob). Values are binary blobs, which
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are always represented as their base64 encoding in the JSON API. Partition
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keys and sort keys are utf8 strings.
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- Triplets are stored in buckets; each bucket stores a separate set of triplets
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- Bucket names and access keys are the same as for accessing the S3 API
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- K2V triplets exist separately from S3 objects. K2V triples don't exist for
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the S3 API, and S3 objects don't exist for the K2V API.
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- Values stored for triples have associated causality information, that enables
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Garage to detect concurrent writes. In case of concurrent writes, Garage
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keeps the concurrent values until a further write supersedes the concurrent
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values. This is the same method as Riak KV implements. The method used is
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based on DVVS (dotted version vector sets), described in the paper "Scalable
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and Accurate Causality Tracking for Eventually Consistent Data Stores", as
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well as [here](https://github.com/ricardobcl/Dotted-Version-Vectors)
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## API Endpoints
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### Operations on single items
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**ReadItem: `GET /<bucket>/<partition key>?sort_key=<sort key>`**
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Query parameters:
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| name | default value | meaning |
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| - | - | - |
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| `sort_key` | **mandatory** | The sort key of the item to read |
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Returns the item with specified partition key and sort key. Values can be
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returned in either of two ways:
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1. a JSON array of base64-encoded values, or `null`'s for tombstones, with
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header `Content-Type: application/json`
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2. in the case where there are no concurrent values, the single present value
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can be returned directly as the response body (or an HTTP 204 NO CONTENT for
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a tombstone), with header `Content-Type: application/octet-stream`
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The choice between return formats 1 and 2 is directed by the `Accept` HTTP header:
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- if the `Accept` header is not present, format 1 is always used
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- if `Accept` contains `application/json` but not `application/octet-stream`,
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format 1 is always used
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- if `Accept` contains `application/octet-stream` but not `application/json`,
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format 2 is used when there is a single value, and an HTTP error 409 (HTTP
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409 CONFLICT) is returned in the case of multiple concurrent values
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(including concurrent tombstones)
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- if `Accept` contains both, format 2 is used when there is a single value, and
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format 1 is used as a fallback in case of concurrent values
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- if `Accept` contains none, HTTP 406 NOT ACCEPTABLE is raised
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Example query:
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```
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GET /my_bucket/mailboxes?sort_key=INBOX HTTP/1.1
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```
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Example response:
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```json
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HTTP/1.1 200 OK
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X-Garage-Causality-Token: opaquetoken123
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Content-Type: application/json
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[
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"b64cryptoblob123",
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"b64cryptoblob'123"
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]
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```
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Example response in case the item is a tombstone:
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```
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HTTP/1.1 200 OK
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X-Garage-Causality-Token: opaquetoken999
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Content-Type: application/json
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[
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null
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]
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```
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Example query 2:
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```
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GET /my_bucket/mailboxes?sort_key=INBOX HTTP/1.1
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Accept: application/octet-stream
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```
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Example response if multiple concurrent versions exist:
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```
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HTTP/1.1 409 CONFLICT
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X-Garage-Causality-Token: opaquetoken123
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Content-Type: application/octet-stream
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```
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Example response in case of single value:
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```
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HTTP/1.1 200 OK
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X-Garage-Causality-Token: opaquetoken123
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Content-Type: application/octet-stream
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cryptoblob123
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```
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Example response in case of a single value that is a tombstone:
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```
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HTTP/1.1 204 NO CONTENT
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X-Garage-Causality-Token: opaquetoken123
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Content-Type: application/octet-stream
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```
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**InsertItem: `PUT /<bucket>/<partition key>?sort_key=<sort_key>`**
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Inserts a single item. This request does not use JSON, the body is sent directly as a binary blob.
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To supersede previous values, the HTTP header `X-Garage-Causality-Token` should
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be set to the causality token returned by a previous read on this key. This
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header can be ommitted for the first writes to the key.
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Example query:
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```
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PUT /my_bucket/mailboxes?sort_key=INBOX HTTP/1.1
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X-Garage-Causality-Token: opaquetoken123
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myblobblahblahblah
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```
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Example response:
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```
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HTTP/1.1 200 OK
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```
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**DeleteItem: `DELETE /<bucket>/<partition key>?sort_key=<sort_key>`**
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Deletes a single item. The HTTP header `X-Garage-Causality-Token` must be set
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to the causality token returned by a previous read on this key, to indicate
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which versions of the value should be deleted. The request will not process if
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`X-Garage-Causality-Token` is not set.
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Example query:
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```
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DELETE /my_bucket/mailboxes?sort_key=INBOX HTTP/1.1
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X-Garage-Causality-Token: opaquetoken123
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```
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Example response:
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```
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HTTP/1.1 204 NO CONTENT
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```
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### Operations on index
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**ReadIndex: `GET /<bucket>?start=<start>&end=<end>&limit=<limit>`**
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Lists all partition keys in the bucket for which some triplets exist, and gives
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for each the number of triplets (or an approximation thereof, this value is
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asynchronously updated, and thus eventually consistent).
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Query parameters:
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| name | default value | meaning |
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| - | - | - |
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| `start` | `null` | First partition key to list, in lexicographical order |
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| `end` | `null` | Last partition key to list (excluded) |
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| `limit` | `null` | Maximum number of partition keys to list |
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The response consists in a JSON object that repeats the parameters of the query and gives the result (see below).
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The listing starts at partition key `start`, or if not specified at the
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smallest partition key that exists. It returns partition keys in increasing
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order and stops when either of the following conditions is met:
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1. if `end` is specfied, the partition key `end` is reached or surpassed (if it
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is reached exactly, it is not included in the result)
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2. if `limit` is specified, `limit` partition keys have been listed
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3. no more partition keys are available to list
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In case 2, and if there are more partition keys to list before condition 1
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triggers, then in the result `more` is set to `true` and `nextStart` is set to
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the first partition key that couldn't be listed due to the limit. In the first
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case (if the listing stopped because of the `end` parameter), `more` is not set
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and the `nextStart` key is not specified.
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Example query:
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```
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GET /my_bucket HTTP/1.1
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```
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Example response:
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```json
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HTTP/1.1 200 OK
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{
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start: null,
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end: null,
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limit: null,
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partition_keys: [
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[ "keys", 3043 ],
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[ "mailbox:INBOX", 42 ],
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[ "mailbox:Junk", 2991 ],
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[ "mailbox:Trash", 10 ],
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[ "mailboxes", 3 ],
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],
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more: false,
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nextStart: null,
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}
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```
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### Operations on batches of items
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**InsertBatch: `POST /<bucket>`**
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Simple insertion and deletion of triplets. The body is just a list of items to
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insert in the following format: `[ "<partition key>", "<sort key>", "<causality
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token>"|null, "<value>"|null ]`.
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The causality token should be the one returned in a previous read request (e.g.
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by ReadItem or ReadBatch), to indicate that this write takes into account the
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values that were returned from these reads, and supersedes them causally. If
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the triple is inserted for the first time, the causality token should be set to
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`null`.
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The value is expected to be a base64-encoded binary blob. The value `null` can
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also be used to delete the triple while preserving causality information: this
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allows to know if a delete has happenned concurrently with an insert, in which
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case both are preserved and returned on reads (see below).
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Partition keys and sort keys are utf8 strings which are stored sorted by
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lexicographical ordering of their binary representation.
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Example query:
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```json
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POST /my_bucket HTTP/1.1
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[
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[ "mailbox:INBOX", "001892831", "opaquetoken321", "b64cryptoblob321updated" ],
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[ "mailbox:INBOX", "001892912", null, "b64cryptoblob444" ],
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[ "mailbox:INBOX", "001892932", "opaquetoken654", null ],
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]
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```
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Example response:
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```
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HTTP/1.1 200 OK
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```
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**ReadBatch: `POST /<bucket>?search`**, or alternatively<br/>
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**ReadBatch: `SEARCH /<bucket>`**
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Batch read of triplets in a bucket.
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The request body is a JSON list of searches, that each specify a range of
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items to get (to get single items, set `single_item` to `true`). A search is a
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JSON struct with the following fields:
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| name | default value | meaning |
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| - | - | - |
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| `partition_key` | **mandatory** | The partition key in which to search |
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| `start` | `null` | The sort key of the first item to read |
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| `end` | `null` | The sort key of the last item to read (excluded) |
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| `limit` | `null` | The maximum number of items to return |
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| `single_item` | `false` | Whether to return only the item with sort key `start` |
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| `conflicts_only` | `false` | Whether to return only items that have several concurrent values |
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| `tombstones` | `false` | Whether or not to return tombstone lines to indicate the presence of old deleted items |
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For each of the searches, triplets are listed and returned separately. The
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semantics of `start`, `end` and `limit` is the same as for ReadIndex. The
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additionnal parameter `single_item` allows to get a single item, whose sort key
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is the one given in `start`. Parameters `conflicts_only` and `tombstones`
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control additional filters on the items that are returned.
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The result is a list of length the number of searches, that consists in for
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each search a JSON object specified similarly to the result of ReadIndex, but
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that lists triples within a partition key.
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The format of returned tuples is as follows: `[ "<sort key>", "<causality
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token>", "<value1>", ...]`, with the following fields:
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- sort key: any unicode string used as a sort key
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- causality token: an opaque token served by the server (generally
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base64-encoded) to be used in subsequent writes to this key
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- value: binary blob, always base64-encoded
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- if several concurrent values exist, they are appended at the end
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- in case of concurrent update and deletion, a `null` is added to the list of concurrent values
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- if the `tombstones` query parameter is set to `true`, tombstones are returned
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for items that have been deleted (this can be usefull for inserting after an
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item that has been deleted, so that the insert is not considered
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concurrent with the delete). Tombstones are returned as tuples in the
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same format with only `null` values
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Example query:
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```json
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POST /my_bucket?search HTTP/1.1
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[
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{
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partition_key: "mailboxes",
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},
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{
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partition_key: "mailbox:INBOX",
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start: "001892831",
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limit: 3,
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},
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{
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partition_key: "keys",
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start: "0",
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single_item: true,
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},
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]
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```
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Example associated response body:
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```json
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HTTP/1.1 200 OK
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[
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{
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partition_key: "mailboxes",
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start: null,
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end: null,
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limit: null,
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conflicts_only: false,
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tombstones: false,
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single_item: false,
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items: [
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[ "INBOX", "opaquetoken123", "b64cryptoblob123", "b64cryptoblob'123" ],
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[ "Trash", "opaquetoken456", "b64cryptoblob456" ],
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[ "Junk", "opaquetoken789", "b64cryptoblob789" ],
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],
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more: false,
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nextStart: null,
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},
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{
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partition_key: "mailbox::INBOX",
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start: "001892831",
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end: null,
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limit: 3,
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conflicts_only: false,
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tombstones: false,
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single_item: false,
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items: [
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[ "001892831", "opaquetoken321", "b64cryptoblob321" ],
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[ "001892832", "opaquetoken654", "b64cryptoblob654" ],
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[ "001892874", "opaquetoken987", "b64cryptoblob987" ],
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],
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more: true,
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nextStart: "001892898",
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},
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{
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partition_key: "keys",
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start: "0",
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end: null,
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conflicts_only: false,
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tombstones: false,
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limit: null,
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single_item: true,
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items: [
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[ "0", "opaquetoken999", "b64binarystuff999" ],
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],
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more: false,
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nextStart: null,
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},
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]
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```
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**DeleteBatch: `POST /<bucket>?delete`**
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Batch deletion of triplets. The request format is the same for `POST
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/<bucket>?search` to indicate items or range of items, except that here they
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are deleted instead of returned, but only the fields `partition_key`, `start`,
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`end`, and `single_item` are supported. Causality information is not given by
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the user: this request will internally list all triplets and write deletion
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markers that supersede all of the versions that have been read.
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This request returns for each series of items to be deleted, the number of
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matching items that have been found and deleted.
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Example query:
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```json
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POST /my_bucket?delete HTTP/1.1
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[
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{
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partition_key: "mailbox:OldMailbox",
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},
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{
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partition_key: "mailbox:INBOX",
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start: "0018928321",
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single_item: true,
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},
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]
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```
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Example response:
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```
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HTTP/1.1 200 OK
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[
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{
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partition_key: "mailbox:OldMailbox",
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start: null,
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end: null,
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single_item: false,
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deleted_items: 35,
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},
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{
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partition_key: "mailbox:INBOX",
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start: "0018928321",
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end: null,
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single_item: true,
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deleted_items: 1,
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},
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]
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```
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## Internals: causality tokens
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The method used is based on DVVS (dotted version vector sets). See:
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- the paper "Scalable and Accurate Causality Tracking for Eventually Consistent Data Stores"
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- <https://github.com/ricardobcl/Dotted-Version-Vectors>
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For DVVS to work, write operations (at each node) must take a lock on the data table.
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