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+++ title = "Design draft" weight = 25 +++
Design draft
WARNING: this documentation is a design draft which was written before Garage's actual implementation. The general principle are similar, but details have not been updated.
Modules
membership/: configuration, membership management (gossip of node's presence and status), ring generation --> what about Serf (used by Consul/Nomad) : https://www.serf.io/? Seems a huge library with many features so maybe overkill/hard to integratemetadata/: metadata managementblocks/: block management, writing, GC and rebalancinginternal/: server to server communication (HTTP server and client that reuses connections, TLS if we want, etc)api/: S3 APIweb/: web management interface
Metadata tables
Objects:
- Hash key: Bucket name (string)
- Sort key: Object key (string)
- Sort key: Version timestamp (int)
- Sort key: Version UUID (string)
- Complete: bool
- Inline: bool, true for objects < threshold (say 1024)
- Object size (int)
- Mime type (string)
- Data for inlined objects (blob)
- Hash of first block otherwise (string)
Having only a hash key on the bucket name will lead to storing all file entries of this table for a specific bucket on a single node. At the same time, it is the only way I see to rapidly being able to list all bucket entries...
Blocks:
- Hash key: Version UUID (string)
- Sort key: Offset of block in total file (int)
- Hash of data block (string)
A version is defined by the existence of at least one entry in the blocks table for a certain version UUID. We must keep the following invariant: if a version exists in the blocks table, it has to be referenced in the objects table. We explicitly manage concurrent versions of an object: the version timestamp and version UUID columns are index columns, thus we may have several concurrent versions of an object. Important: before deleting an older version from the objects table, we must make sure that we did a successfull delete of the blocks of that version from the blocks table.
Thus, the workflow for reading an object is as follows:
- Check permissions (LDAP)
- Read entry in object table. If data is inline, we have its data, stop here. -> if several versions, take newest one and launch deletion of old ones in background
- Read first block from cluster. If size <= 1 block, stop here.
- Simultaneously with previous step, if size > 1 block: query the Blocks table for the IDs of the next blocks
- Read subsequent blocks from cluster
Workflow for PUT:
- Check write permission (LDAP)
- Select a new version UUID
- Write a preliminary entry for the new version in the objects table with complete = false
- Send blocks to cluster and write entries in the blocks table
- Update the version with complete = true and all of the accurate information (size, etc)
- Return success to the user
- Launch a background job to check and delete older versions
Workflow for DELETE:
- Check write permission (LDAP)
- Get current version (or versions) in object table
- Do the deletion of those versions NOT IN A BACKGROUND JOB THIS TIME
- Return succes to the user if we were able to delete blocks from the blocks table and entries from the object table
To delete a version:
- List the blocks from Cassandra
- For each block, delete it from cluster. Don't care if some deletions fail, we can do GC.
- Delete all of the blocks from the blocks table
- Finally, delete the version from the objects table
Known issue: if someone is reading from a version that we want to delete and the object is big, the read might be interrupted. I think it is ok to leave it like this, we just cut the connection if data disappears during a read.
("Soit P un problème, on s'en fout est une solution à ce problème")
Block storage on disk
Blocks themselves:
- file path = /blobs/(first 3 hex digits of hash)/(rest of hash)
Reverse index for GC & other block-level metadata:
- file path = /meta/(first 3 hex digits of hash)/(rest of hash)
- map block hash -> set of version UUIDs where it is referenced
Usefull metadata:
- list of versions that reference this block in the Casandra table, so that we can do GC by checking in Cassandra that the lines still exist
- list of other nodes that we know have acknowledged a write of this block, usefull in the rebalancing algorithm
Write strategy: have a single thread that does all write IO so that it is serialized (or have several threads that manage independent parts of the hash space). When writing a blob, write it to a temporary file, close, then rename so that a concurrent read gets a consistent result (either not found or found with whole content).
Read strategy: the only read operation is get(hash) that returns either the data or not found (can do a corruption check as well and return corrupted state if it is the case). Can be done concurrently with writes.
Internal API:
- get(block hash) -> ok+data/not found/corrupted
- put(block hash & data, version uuid + offset) -> ok/error
- put with no data(block hash, version uuid + offset) -> ok/not found plz send data/error
- delete(block hash, version uuid + offset) -> ok/error
GC: when last ref is deleted, delete block. Long GC procedure: check in Cassandra that version UUIDs still exist and references this block.
Rebalancing: takes as argument the list of newly added nodes.
- List all blocks that we have. For each block:
- If it hits a newly introduced node, send it to them. Use put with no data first to check if it has to be sent to them already or not. Use a random listing order to avoid race conditions (they do no harm but we might have two nodes sending the same thing at the same time thus wasting time).
- If it doesn't hit us anymore, delete it and its reference list.
Only one balancing can be running at a same time. It can be restarted at the beginning with new parameters.
Membership management
Two sets of nodes:
- set of nodes from which a ping was recently received, with status: number of stored blocks, request counters, error counters, GC%, rebalancing% (eviction from this set after say 30 seconds without ping)
- set of nodes that are part of the system, explicitly modified by the operator using the web UI (persisted to disk), is a CRDT using a version number for the value of the whole set
Thus, three states for nodes:
- healthy: in both sets
- missing: not pingable but part of desired cluster
- unused/draining: currently present but not part of the desired cluster, empty = if contains nothing, draining = if still contains some blocks
Membership messages between nodes:
- ping with current state + hash of current membership info -> reply with same info
- send&get back membership info (the ids of nodes that are in the two sets): used when no local membership change in a long time and membership info hash discrepancy detected with first message (passive membership fixing with full CRDT gossip)
- inform of newly pingable node(s) -> no result, when receive new info repeat to all (reliable broadcast)
- inform of operator membership change -> no result, when receive new info repeat to all (reliable broadcast)
Ring: generated from the desired set of nodes, however when doing read/writes on the ring, skip nodes that are known to be not pingable. The tokens are generated in a deterministic fashion from node IDs (hash of node id + token number from 1 to K). Number K of tokens per node: decided by the operator & stored in the operator's list of nodes CRDT. Default value proposal: with node status information also broadcast disk total size and free space, and propose a default number of tokens equal to 80%Free space / 10Gb. (this is all user interface)
Constants
- Block size: around 1MB ? --> Exoscale use 16MB chunks
- Number of tokens in the hash ring: one every 10Gb of allocated storage
- Threshold for storing data directly in Cassandra objects table: 1kb bytes (maybe up to 4kb?)
- Ping timeout (time after which a node is registered as unresponsive/missing): 30 seconds
- Ping interval: 10 seconds
- ??