Move doc files here from Deuxfleur's site repo

doc/Internals.md

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+#### 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)

doc/Quickstart.md

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+# Quickstart on an existing deployment
+
+First, chances are that your garage deployment is secured by TLS.
+All your commands must be prefixed with their certificates.
+I will define an alias once and for all to ease future commands.
+Please adapt the path of the binary and certificates to your installation!
+
+```
+alias grg="/garage/garage --ca-cert /secrets/garage-ca.crt --client-cert /secrets/garage.crt --client-key /secrets/garage.key"
+```
+
+Now we can check that everything is going well by checking our cluster status:
+
+```
+grg status
+```
+
+Don't forget that `help` command and `--help` subcommands can help you anywhere, the CLI tool is self-documented! Two examples:
+
+```
+grg help
+grg bucket allow --help
+```
+
+Fine, now let's create a bucket (we imagine that you want to deploy nextcloud):
+
+```
+grg bucket create nextcloud-bucket
+```
+
+Check that everything went well:
+
+```
+grg bucket list
+grg bucket info nextcloud-bucket
+```
+
+Now we will generate an API key to access this bucket.
+Note that API keys are independent of buckets: one key can access multiple buckets, multiple keys can access one bucket.
+
+Now, let's start by creating a key only for our PHP application:
+
+```
+grg key new --name nextcloud-app-key
+```
+
+You will have the following output (this one is fake, `key_id` and `secret_key` were generated with the openssl CLI tool):
+
+```
+Key { key_id: "GK3515373e4c851ebaad366558", secret_key: "7d37d093435a41f2aab8f13c19ba067d9776c90215f56614adad6ece597dbb34", name: "nextcloud-app-key", name_timestamp: 1603280506694, deleted: false, authorized_buckets: [] }
+```
+
+Check that everything works as intended (be careful, info works only with your key identifier and not with its friendly name!):
+
+```
+grg key list
+grg key info GK3515373e4c851ebaad366558
+```
+
+Now that we have a bucket and a key, we need to give permissions to the key on the bucket!
+
+```
+grg bucket allow --read --write nextcloud-bucket --key GK3515373e4c851ebaad366558
+```
+
+You can check at any times allowed keys on your bucket with:
+
+```
+grg bucket info nextcloud-bucket
+```
+
+Now, let's move to the S3 API!
+We will use the `s3cmd` CLI tool.
+You can install it via your favorite package manager.
+Otherwise, check [their website](https://s3tools.org/s3cmd)
+
+We will configure `s3cmd` with its interactive configuration tool, be careful not all endpoints are implemented!
+Especially, the test run at the end does not work (yet).
+
+```
+$ s3cmd --configure
+
+Enter new values or accept defaults in brackets with Enter.
+Refer to user manual for detailed description of all options.
+
+Access key and Secret key are your identifiers for Amazon S3. Leave them empty for using the env variables.
+Access Key: GK3515373e4c851ebaad366558
+Secret Key: 7d37d093435a41f2aab8f13c19ba067d9776c90215f56614adad6ece597dbb34
+Default Region [US]: garage
+
+Use "s3.amazonaws.com" for S3 Endpoint and not modify it to the target Amazon S3.
+S3 Endpoint [s3.amazonaws.com]: garage.deuxfleurs.fr
+
+Use "%(bucket)s.s3.amazonaws.com" to the target Amazon S3. "%(bucket)s" and "%(location)s" vars can be used
+if the target S3 system supports dns based buckets.
+DNS-style bucket+hostname:port template for accessing a bucket [%(bucket)s.s3.amazonaws.com]: garage.deuxfleurs.fr
+
+Encryption password is used to protect your files from reading
+by unauthorized persons while in transfer to S3
+Encryption password: 
+Path to GPG program [/usr/bin/gpg]: 
+
+When using secure HTTPS protocol all communication with Amazon S3
+servers is protected from 3rd party eavesdropping. This method is
+slower than plain HTTP, and can only be proxied with Python 2.7 or newer
+Use HTTPS protocol [Yes]: 
+
+On some networks all internet access must go through a HTTP proxy.
+Try setting it here if you can't connect to S3 directly
+HTTP Proxy server name: 
+
+New settings:
+  Access Key: GK3515373e4c851ebaad366558
+  Secret Key: 7d37d093435a41f2aab8f13c19ba067d9776c90215f56614adad6ece597dbb34
+  Default Region: garage
+  S3 Endpoint: garage.deuxfleurs.fr
+  DNS-style bucket+hostname:port template for accessing a bucket: garage.deuxfleurs.fr
+  Encryption password: 
+  Path to GPG program: /usr/bin/gpg
+  Use HTTPS protocol: True
+  HTTP Proxy server name: 
+  HTTP Proxy server port: 0
+
+Test access with supplied credentials? [Y/n] n
+
+Save settings? [y/N] y
+Configuration saved to '/home/quentin/.s3cfg'
+```
+
+Now, if everything works, the following commands should work:
+
+```
+echo hello world > hello.txt
+s3cmd put hello.txt s3://nextcloud-bucket
+s3cmd ls s3://nextcloud-bucket
+s3cmd rm s3://nextcloud-bucket/hello.txt
+```
+
+That's all for now!
+

doc/Related Work.md

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+## Context
+
+Data storage is critical: it can lead to data loss if done badly and/or on hardware failure.
+Filesystems + RAID can help on a single machine but a machine failure can put the whole storage offline.
+Moreover, it put a hard limit on scalability. Often this limit can be pushed back far away by buying expensive machines.
+But here we consider non specialized off the shelf machines that can be as low powered and subject to failures as a raspberry pi.
+
+Distributed storage may help to solve both availability and scalability problems on these machines.
+Many solutions were proposed, they can be categorized as block storage, file storage and object storage depending on the abstraction they provide.
+
+## Related work
+
+Block storage is the most low level one, it's like exposing your raw hard drive over the network.
+It requires very low latencies and stable network, that are often dedicated.
+However it provides disk devices that can be manipulated by the operating system with the less constraints: it can be partitioned with any filesystem, meaning that it supports even the most exotic features.
+We can cite [iSCSI](https://en.wikipedia.org/wiki/ISCSI) or [Fibre Channel](https://en.wikipedia.org/wiki/Fibre_Channel).
+Openstack Cinder proxy previous solution to provide an uniform API.
+
+File storage provides a higher abstraction, they are one filesystem among others, which means they don't necessarily have all the exotic features of every filesystem.
+Often, they relax some POSIX constraints while many applications will still be compatible without any modification.
+As an example, we are able to run MariaDB (very slowly) over GlusterFS...
+We can also mention CephFS (read [RADOS](https://ceph.com/wp-content/uploads/2016/08/weil-rados-pdsw07.pdf) whitepaper), Lustre, LizardFS, MooseFS, etc.
+OpenStack Manila proxy previous solutions to provide an uniform API.
+
+Finally object storages provide the highest level abstraction.
+They are the testimony that the POSIX filesystem API is not adapted to distributed filesystems.
+Especially, the strong concistency has been dropped in favor of eventual consistency which is way more convenient and powerful in presence of high latencies and unreliability.
+We often read about S3 that pioneered the concept that it's a filesystem for the WAN.
+Applications must be adapted to work for the desired object storage service.
+Today, the S3 HTTP REST API acts as a standard in the industry.
+However, Amazon S3 source code is not open but alternatives were proposed.
+We identified Minio, Pithos, Swift and Ceph.
+Minio/Ceph enforces a total order, so properties similar to a (relaxed) filesystem.
+Swift and Pithos are probably the most similar to AWS S3 with their consistent hashing ring.
+However Pithos is not maintained anymore. More precisely the company that published Pithos version 1 has developped a second version 2 but has not open sourced it.
+Some tests conducted by the [ACIDES project](https://acides.org/) have shown that Openstack Swift consumes way more resources (CPU+RAM) that we can afford. Furthermore, people developing Swift have not designed their software for geo-distribution.
+
+There were many attempts in research too. I am only thinking to [LBFS](https://pdos.csail.mit.edu/papers/lbfs:sosp01/lbfs.pdf) that was used as a basis for Seafile. But none of them have been effectively implemented yet.