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-title="Doc Post"
-date=2018-08-20
-+++
-
-# 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)