.. _swh-archiver-blueprint:

Archiver blueprint
==================

The Software Heritage (SWH) Archiver is responsible for backing up SWH
objects as to reduce the risk of losing them.

Currently, the archiver only deals with content objects (i.e., those
referenced by the content table in the DB and stored in the SWH object
storage). The database itself is lively replicated by other means.

Requirements
------------

Peer-to-peer topology
~~~~~~~~~~~~~~~~~~~~~

Every storage involved in the archival process can be used as a source
or a destination for the archival, depending on the blobs it contains. A
retention policy specifies the minimum number of copies that are
required to be "safe".

Although the servers are totally equals the coordination of which
content should be copied and from where to where is centralized.

Append-only archival
~~~~~~~~~~~~~~~~~~~~

The archiver treats involved storages as append-only storages. The
archiver never deletes any object. If removals are needed, they will be
dealt with by other means.

Asynchronous archival
~~~~~~~~~~~~~~~~~~~~~

Periodically (e.g., via cron), the archiver runs, produces a list of
objects that need to have more copies, and starts copying them over. The
decision of which storages are choosen to be sources and destinations
are not up to the storages themselves.

Very likely, during any given archival run, other new objects will be
added to storages; it will be the responsibility of *future* archiver
runs, and not the current one, to copy new objects over if needed.

Integrity at archival time
~~~~~~~~~~~~~~~~~~~~~~~~~~

Before archiving objects, the archiver performs suitable integrity
checks on them. For instance, content objects are verified to ensure
that they can be decompressed and that their content match their
(checksum-based) unique identifiers. Corrupt objects will not be
archived and suitable errors reporting about the corruption will be
emitted.

Note that archival-time integrity checks are *not meant to replace
periodic integrity checks*.

Parallel archival
~~~~~~~~~~~~~~~~~

Once the list of objects to be archived has been identified, it SHOULD
be possible to archive objects in parallel w.r.t. one another.

Persistent archival status
~~~~~~~~~~~~~~~~~~~~~~~~~~

The archiver maintains a mapping between objects and their storage
locations. Locations are the set {master, slave\_1, ..., slave\_n}.

Each pair is also associated to the following information:

-  **status**: 4-state: *missing* (copy not present at destination),
   *ongoing* (copy to destination ongoing), *present* (copy present at
   destination), *corrupted* (content detected as corrupted during an
   archival).
-  **mtime**: timestamp of last status change. This is either the
   destination archival time (when status=present), or the timestamp of
   the last archival request (status=ongoing); the timestamp is
   undefined when status=missing.

Architecture
------------

The archiver is comprised of the following software components:

-  archiver director
-  archiver worker
-  archiver copier

Archiver director
~~~~~~~~~~~~~~~~~

The archiver director is run periodically, e.g., via cron.

Runtime parameters:

-  execution periodicity (external)
-  retention policy
-  archival max age
-  archival batch size

At each execution the director:

1. for each object: retrieve its archival status
2. for each object that has fewer copies than those requested by the
   retention policy:
3. mark object as needing archival
4. group objects in need of archival in batches of
   ``archival batch size``
5. for each batch:
6. spawn an archive worker on the whole batch (e.g., submitting the
   relevant celery task)

Archiver worker
~~~~~~~~~~~~~~~

The archiver is executed on demand (e.g., by a celery worker) to archive
a given set of objects.

Runtime parameters:

-  objects to archive
-  archival policies (retention & archival max age)

At each execution a worker:

1.  for each object in the batch
2.  check that the object still need to be archived (#present copies <
    retention policy)
3.  if an object has status=ongoing but the elapsed time from task
    submission is less than the *archival max age*, it counts as present
    (as we assume that it will be copied in the near future). If the
    delay is elapsed (still with status ongoing), it counts as a missing
    copy.
4.  for each object to archive:
5.  retrieve current archive status for all destinations
6.  create a map noting where the object is present and where it can be
    copied
7.  Randomly choose couples (source, destination), where destinations
    are all differents, to make enough copies
8.  for each (content, source, destination):
9.  Join the contents by key (source, destination) to have a map
    {(source, destination) -> [contents]}
10. for each (source, destination) -> contents
11. for each content in contents, check its integrity on the source
    storage

    -  if the object is corrupted or missing

       -  update its status in the database
       -  remove it from the current contents list

12. start the copy of the batches by launching for each transfer tuple a
    copier

    -  if an error occurred on one of the content that should have been
       valid, consider the whole batch as a failure.

13. set status=present and mtime=now for each successfully copied object

Note that:

-  In case multiple jobs where tasked to archive the same overlapping
   objects, step (1) might decide that some/all objects of this batch no
   longer needs to be archived.

-  Due to parallelism, it is possible that the same objects will be
   copied over at the same time by multiple workers. Also, the same
   object could end up having more copies than the minimal number
   required.

Archiver copier
~~~~~~~~~~~~~~~

The copier is run on demand by archiver workers, to transfer file
batches from a given source to a given destination.

The copier transfers files one by one. The copying process is atomic
with a file granularity (i.e., individual files might be visible on the
destination before *all* files have been transferred) and ensures that
*concurrent transfer of the same files by multiple copier instances do
not result in corrupted files*. Note that, due to this and the fact that
timestamps are updated by the worker, all files copied in the same batch
will have the same mtime even though the actual file creation times on a
given destination might differ.

The copier is implemented using the ObjStorage API for the sources and
destinations.

DB structure
------------

Postgres SQL definitions for the archival status:

::

    -- Those names are sample of archives server names
    CREATE TYPE archive_id AS ENUM (
      'uffizi',
      'banco'
    );

    CREATE TABLE archive (
      id   archive_id PRIMARY KEY,
      url  TEXT
    );

    CREATE TYPE archive_status AS ENUM (
      'missing',
      'ongoing',
      'present',
      'corrupted'
    );

    CREATE TABLE content_archive (
      content_id sha1 unique,
      copies jsonb
    );

Where the content\_archive.copies field is of type jsonb. It contains
content's presence (or absence) in storages. A content being represented
by its signature (sha1)

::

    {
        "$schema": "http://json-schema.org/schema#",
        "title": "Copies data",
        "description": "Data about the presence of a content into the storages",
        "type": "object",
        "Patternproperties": {
            "^[a-zA-Z1-9]+$": {
                "description": "archival status for the server named by key",
                "type": "object",
                "properties": {
                    "status": {
                        "description": "Archival status on this copy",
                        "type": "string",
                        "enum": ["missing", "ongoing", "present", "corrupted"]
                    },
                    "mtime": {
                        "description": "Last time of the status update",
                        "type": "float"
                    }
                }
            }
        },
        "additionalProperties": false
    }