Reorganize documentation for new website (#213)

This PR should be merged after the new website is deployed.

- [x] Rename files
- [x] Add front matter section to all `.md` files in the book (necessary for Zola)
- [x] Change all internal links to use Zola's linking system that checks broken links
- [x] Some updates to documentation contents and organization

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/213
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

1 files changed, 165 insertions, 0 deletions

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+title = "Design draft"
+weight = 25
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+
+**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://doi.org/10.1145/1374596.1374606) [[pdf](https://ceph.com/assets/pdfs/weil-rados-pdsw07.pdf)]