pict-rs/docs/postgres-planning.md

Planning for implementing a postgres repo for pict-rs

Technology

I've identified these crates as useful for achieving a reasonable postgres experience

• diesel-async
• refinery
• deadpool
• tokio-postgres

tokio-postgres will actually do the work of talking to postgres in all cases. diesel-async can use tokio-postgres to execute queries. refinery can use tokio-postgres to run migrations. deadpool can pool tokio-postgres connections.

I've chosen this stack specifically to avoid depending on libpq, the c implementation of a postgres client. This is not because it's written in C. 99.9% of postgres client libraries use libpq to great success. It is to keep the build process for pict-rs simple. Sticking with a full stack of rust means that only a rust compiler is required to build pict-rs.

Plan

pict-rs isolates different concepts between different "Repo" traits. There's a single top-level FullRepo that depends on the others to ensure everything gets implemented properly. Since there's only been one repo implementation so far, it's not optimized for network databases and some things are less efficient than they could be.

HashRepo

This is a little complicated because one of the things a HashRepo can do is return a stream of hashes from the repo. This can likely be implemented as a batch-retrieval operation that fetches 1000 hashes at once and then drains them on each call to poll_next

methods:

• size
• hashes
• create
• update_identifier
• identifier
• relate_variant_identifier
• variant_identifier
• variants
• remove_variant
• relate_motion_identifier
• motion_identifier
• cleanup

CREATE TABLE hashes (
    hash BYTEA PRIMARY KEY,
    identifer TEXT NOT NULL,
    motion_identifier TEXT,
);


CREATE TABLE variants (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    hash BYTEA REFERENCES hashes(hash) ON DELETE CASCADE,
    variant TEXT NOT NULL,
    identifier TEXT NOT NULL
);


CREATE UNIQUE INDEX hash_variant_index ON variants (hash, variant);

AliasRepo

Used to relate Aliases to Hashes, and to relate Delete Tokens to Aliases. Hashes are always bytes, but could be hex-encoded. postgres's bytea type can work with hex-encoding on storage and retrieval so maybe this can be used. Delete Tokens are not always UUIDs, even though they have been UUIDs in all recent versions of pict-rs.

methods:

• create
• delete_token
• hash
• for_hash
• cleanup

CREATE TABLE aliases (
    alias VARCHAR(30) PRIMARY KEY,
    hash BYTEA NOT NULL REFERENCES hashes(hash) ON DELETE CASCADE,
    delete_token VARCHAR(30) NOT NULL
);

SettingsRepo

This is used for generic server-level storage. The file & object stores keep their current path generator values here. This is also used in some migrations to mark completion.

methods:

• set
• get
• remove

could be a simple table with String key & values

pict-rs currently treats the value here as Bytes, so it could either be hex-encoded or translated to a string

CREATE TABLE settings (
    key VARCHAR(80) PRIMARY KEY,
    value VARCHAR(80) NOT NULL
);

IdentifierRepo

Used to relate details (image metadata) to identifiers (image paths). Identifiers are currently treated as bytes, so may need hex-encoding to store in the database. They should be valid strings in most environments, so it might be possible to drop the bytes requirement & instead have a string requirement.

methods:

• relate_details
• details
• cleanup

CREATE TABLE details (
    identifier TEXT PRIMARY KEY,
    details JSONB NOT NULL,
);

QueueRepo

This is going to be the troublesome table. It represents jobs that will be processed. Jobs are pushed as Bytes, but at a higher level are actually JSON strings. The QueueRepo API could be updated to take T: Serizlie as input rather than bytes, and then we can store it as JSONB. With the current API, the repo doesn't need to know the shape of a job, and maybe that is a benefit. We should take care in the future not to query on the contents of the job.

methods:

• requeue_in_progress
• push
• pop

CREATE TYPE job_status AS ENUM ('new', 'running');


CREATE TABLE queue (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    queue VARCHAR(30) NOT NULL,
    job JSONB NOT NULL,
    worker_id VARCHAR(30),
    status job_status NOT NULL DEFAULT 'new',
    queue_time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
);


CREATE INDEX queue_worker_id_index ON queue INCLUDE worker_id;
CREATE INDEX queue_status_index ON queue INCLUDE status;

claiming a job can be

DELETE FROM queue WHERE worker_id = '$WORKER_ID';


UPDATE queue SET status = 'running', worker_id = '$WORKER_ID'
WHERE id = (
    SELECT id
    FROM queue
    WHERE status = 'new'
    ORDER BY queue_time ASC
    FOR UPDATE SKIP LOCKED
    LIMIT 1
)
returning *;

notifying pict-rs of a ready job could be

CREATE OR REPLACE FUNCTION queue_status_notify()
	RETURNS trigger AS
$$
BEGIN
	PERFORM pg_notify('queue_status_channel', NEW.id::text);
	RETURN NEW;
END;
$$ LANGUAGE plpgsql;


CREATE TRIGGER queue_status
	AFTER INSERT OR UPDATE OF status
	ON queue
	FOR EACH ROW
EXECUTE PROCEDURE queue_status_notify();

Postgres queue implementation from this blog post: https://webapp.io/blog/postgres-is-the-answer/

MigrationRepo

This is used for migrating from local storage to object storage. It keeps track of which identifiers have been migrated, and on a successful migration, it is fully cleared.

methods:

• is_continuing_migration
• mark_migrated
• is_migrated
• clear

CREATE TABLE migrations (
    identifier TEXT PRIMARY KEY,
);

ProxyRepo

This is used for keeping track of URLs that map to Aliases for proxied media.

methods:

• relate_url
• related
• remove_relation

CREATE TABLE proxies (
    url PRIMARY KEY,
    alias VARCHAR(30) NOT NULL REFERENCES aliases(alias)
);

AliasAccessRepo

This is used for keeping track of aliases that are "cached" in pict-rs and can be safely removed when they are not useful to keep around. This might be able to piggyback on the aliases table or the proxies table.

methods:

• accessed
• older_aliases
• remove_access

ALTER TABLE aliases ADD COLUMN accessed TIMESTAMP;

or

ALTER TABLE proxies ADD COLUMN accessed TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;

VariantAccessRepo

This is used for keeping track of access times for variants of an image to enable freeing up space from seldom-accessed variants. This might be able to piggyback on the variants table.

methods:

• accessed
• contains_variant
• older_variants
• remove_access

ALTER TABLE variants ADD COLUMN accessed TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;