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diff --git a/content/documentation/working-documents/design_draft.md b/content/documentation/working-documents/design_draft.md new file mode 100644 index 0000000..90c2d1c --- /dev/null +++ b/content/documentation/working-documents/design_draft.md @@ -0,0 +1,167 @@ ++++ +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 integrate +- `metadata/`: metadata management +- `blocks/`: block management, writing, GC and rebalancing +- `internal/`: server to server communication (HTTP server and client that reuses connections, TLS if we want, etc) +- `api/`: S3 API +- `web/`: 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: + +1. Check permissions (LDAP) +2. 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 +3. Read first block from cluster. If size <= 1 block, stop here. +4. Simultaneously with previous step, if size > 1 block: query the Blocks table for the IDs of the next blocks +5. Read subsequent blocks from cluster + +Workflow for PUT: + +1. Check write permission (LDAP) +2. Select a new version UUID +3. Write a preliminary entry for the new version in the objects table with complete = false +4. Send blocks to cluster and write entries in the blocks table +5. Update the version with complete = true and all of the accurate information (size, etc) +6. Return success to the user +7. Launch a background job to check and delete older versions + +Workflow for DELETE: + +1. Check write permission (LDAP) +2. Get current version (or versions) in object table +3. Do the deletion of those versions NOT IN A BACKGROUND JOB THIS TIME +4. 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: + +1. List the blocks from Cassandra +2. For each block, delete it from cluster. Don't care if some deletions fail, we can do GC. +3. Delete all of the blocks from the blocks table +4. 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 +- ?? + +#### Links + +- CDC: <https://www.usenix.org/system/files/conference/atc16/atc16-paper-xia.pdf> +- Erasure coding: <http://web.eecs.utk.edu/~jplank/plank/papers/CS-08-627.html> +- [Openstack Storage Concepts](https://docs.openstack.org/arch-design/design-storage/design-storage-concepts.html) +- [RADOS](https://ceph.com/wp-content/uploads/2016/08/weil-rados-pdsw07.pdf) |