5 ms·

JSON Changelog with SQLite

113 points by iffycan 8y ago

Scaevolus 8y ago
Neat! If you wanted to extend this more (string diffs, non-json encodings), a custom function would be the next logical step. In Python it might be: import sqlite3 def delta(*args): ret = {} for name, old, new in zip(args[::3], args[1::3], args[2::3]): if old != new: ret[name] = old if ret: return ret db = sqlite3.connect(':memory:') db.create_function('delta1', delta, 3) db.create_function('delta2', delta, 6) db.create_function('delta3', delta, 9) print db.execute('select delta2("b", "20", "20", "c", 3.4, 3.6)').fetchone() >> (u'{"c": 3.4}',)
- swuecho 8y ago
  in python 3.6 import sqlite3 import json def delta(*args): ret = {} for name, old, new in zip(args[::3], args[1::3], args[2::3]): if old != new: ret[name] = old if ret: return json.dumps(ret) db = sqlite3.connect(':memory:') db.create_function('delta1',3, delta) db.create_function('delta2',6, delta) db.create_function('delta3',9, delta) print(db.execute('select delta2("b", 20, 20, "c", 3.4, 3.6)').fetchone()) # ('{"c": 3.4}',)
derefr 8y ago
I've seen this pattern a couple of times (e.g. for broadcasting changes using Postgres's LISTEN/NOTIFY.) Each time, I wonder why people are recreating the database's journal, within the database, where this meta-journal will itself be getting journaled. Why not just consume the actual DB journal? I'm not sure how that'd work for SQLite—it'd probably require a hack of some sort, since SQLite's journal is an ephemeral part of the DB file. But for more full-fledged RDBMSes, this would just be a matter of hijacking a streaming-replication-based backup system to act as a store of record for events. E.g., in the case of Postgres, you'd just have an ETL pipeline pointed at your WAL-E bucket, parsing through your WAL files either to load into a separate OLAP store, or for stateless map-reduction whenever you need to answer a question.
- anarazel 8y ago
  > Why not just consume the actual DB journal? It's decidedly non-trivial to do so, as usually the journal doesn't contain all the information to do so. Journal writes are often a bottleneck so DB engines try to restrict their contents to just the necessary parts. Typically it'll e.g. not contain information about table schemas etc. You can do so, see e.g. postgresql's logical decoding, but it's plenty of additional work.
  - derefr 8y ago
    > Journal writes are often a bottleneck The point I was trying to make is that the alternative—writing all your DB activity into the DB—means that you're already, 100% guaranteed, creating more overhead than the heaviest possible journal-safety properties would create. You're writing unabridged journal entries into the DB, where they then get even more journal attached. There is no world where it makes more sense to write a copy of everything you do into an "everything I did" table, than it does to use an append-only / immutable / log-centric DBMS (where "Postgres with an ETL pipeline pointed at its WAL logs" is a rather hacky immutable DBMS.) If you know, before you ever start coding, that you will need to know everything that ever happened, writing everything twice—when your DBMS is already writing everything twice—is a rather silly way to choose to go about solving your problems. You're already in a world where you need different guarantees than a traditional (ephemeral) RDBMS gives you; you just haven't realized it yet.
    - cm2187 8y ago
      Perhaps it’s a dev vs ops problem. Only ops see the journal, understand it, access it. If the dev want a journal they end up building their own.
      - WorldMaker 8y ago
        It's also a public versus private API issue. Most databases consider their journal a private implementation detail and don't recommend programming directly against it, as implementation can change as the developers change internal details from database software version to version, or even on a runtime whim as the system's query optimizer/write buffer state/etc needs change over time in some of the databases. For instance, Microsoft SQL Server has a built-in public change tracking API, which you can tell is a leaky abstraction over top of the journal, but SQL Server makes an API guarantee when using that specific change tracking API whereas (at least the last time I looked) it will never make API guarantees about anything lower level than that that accesses its journal. The Change Tracking API documentation even warns you that it locks the journal into certain deoptimizations and that there are clear performance trade-offs, which also hints that using the lower level journal API more directly would likely hit optimization problems and data loss issues. As a dev, it's rarely a good idea to program against APIs your vendor intentionally deems to be private/implementation details that can change between versions/may be unstable in runtime operation. If the database doesn't offer a public journaling API, then yes, sadly, sometimes the best or at least least-worst answer is to build a sub-par journal inside the database itself, with of course its own tradeoffs in performance.
cannadayr 8y ago
I solved this problem by wrapping sqldiff (included w/ sqlite src) as a custom git diff driver. https://github.com/cannadayr/git-sqlite https://github.com/cannadayr/git-sqlite
groue 8y ago
Related article: "Automatic Undo/Redo Using SQLite" https://www.sqlite.org/undoredo.html https://www.sqlite.org/undoredo.html
- iffycan 8y ago
  Thank you for posting this. A former version of my post included a reference to this because it's a great read, too.
radiospiel 8y ago
The sqlite sessions extension seems to be a good foundation for getting changelogs out of a sqlite database. It is sadly not too well known: https://www.sqlite.org/sessionintro.html https://www.sqlite.org/sessionintro.html
janemanos 8y ago
ArangoDB also has a first tool to listen to changes in the database and execute actions or queries. It is limited to a single node. Question is how scalable all of these tools are https://www.arangodb.com/2017/03/arangochair-tool-listening-changes-arangodb/ https://www.arangodb.com/2017/03/arangochair-tool-listening-...

113 points by iffycan 8y ago

Neat! If you wanted to extend this more (string diffs, non-json encodings), a custom function would be the next logical step. In Python it might be: import sqlite3 def delta(*args): ret = {} for name, old, new in zip(args[::3], args[1::3], args[2::3]): if old != new: ret[name] = old if ret: return ret db = sqlite3.connect(':memory:') db.create_function('delta1', delta, 3) db.create_function('delta2', delta, 6) db.create_function('delta3', delta, 9) print db.execute('select delta2("b", "20", "20", "c", 3.4, 3.6)').fetchone() >> (u'{"c": 3.4}',)

swuecho 8y ago
in python 3.6 import sqlite3 import json def delta(*args): ret = {} for name, old, new in zip(args[::3], args[1::3], args[2::3]): if old != new: ret[name] = old if ret: return json.dumps(ret) db = sqlite3.connect(':memory:') db.create_function('delta1',3, delta) db.create_function('delta2',6, delta) db.create_function('delta3',9, delta) print(db.execute('select delta2("b", 20, 20, "c", 3.4, 3.6)').fetchone()) # ('{"c": 3.4}',)

derefr 8y ago

I've seen this pattern a couple of times (e.g. for broadcasting changes using Postgres's LISTEN/NOTIFY.) Each time, I wonder why people are recreating the database's journal, within the database, where this meta-journal will itself be getting journaled. Why not just consume the actual DB journal? I'm not sure how that'd work for SQLite—it'd probably require a hack of some sort, since SQLite's journal is an ephemeral part of the DB file. But for more full-fledged RDBMSes, this would just be a matter of hijacking a streaming-replication-based backup system to act as a store of record for events. E.g., in the case of Postgres, you'd just have an ETL pipeline pointed at your WAL-E bucket, parsing through your WAL files either to load into a separate OLAP store, or for stateless map-reduction whenever you need to answer a question.

anarazel 8y ago
> Why not just consume the actual DB journal? It's decidedly non-trivial to do so, as usually the journal doesn't contain all the information to do so. Journal writes are often a bottleneck so DB engines try to restrict their contents to just the necessary parts. Typically it'll e.g. not contain information about table schemas etc. You can do so, see e.g. postgresql's logical decoding, but it's plenty of additional work.
- derefr 8y ago
  > Journal writes are often a bottleneck The point I was trying to make is that the alternative—writing all your DB activity into the DB—means that you're already, 100% guaranteed, creating more overhead than the heaviest possible journal-safety properties would create. You're writing unabridged journal entries into the DB, where they then get even more journal attached. There is no world where it makes more sense to write a copy of everything you do into an "everything I did" table, than it does to use an append-only / immutable / log-centric DBMS (where "Postgres with an ETL pipeline pointed at its WAL logs" is a rather hacky immutable DBMS.) If you know, before you ever start coding, that you will need to know everything that ever happened, writing everything twice—when your DBMS is already writing everything twice—is a rather silly way to choose to go about solving your problems. You're already in a world where you need different guarantees than a traditional (ephemeral) RDBMS gives you; you just haven't realized it yet.
  - cm2187 8y ago
    Perhaps it’s a dev vs ops problem. Only ops see the journal, understand it, access it. If the dev want a journal they end up building their own.
    - WorldMaker 8y ago
      It's also a public versus private API issue. Most databases consider their journal a private implementation detail and don't recommend programming directly against it, as implementation can change as the developers change internal details from database software version to version, or even on a runtime whim as the system's query optimizer/write buffer state/etc needs change over time in some of the databases. For instance, Microsoft SQL Server has a built-in public change tracking API, which you can tell is a leaky abstraction over top of the journal, but SQL Server makes an API guarantee when using that specific change tracking API whereas (at least the last time I looked) it will never make API guarantees about anything lower level than that that accesses its journal. The Change Tracking API documentation even warns you that it locks the journal into certain deoptimizations and that there are clear performance trade-offs, which also hints that using the lower level journal API more directly would likely hit optimization problems and data loss issues. As a dev, it's rarely a good idea to program against APIs your vendor intentionally deems to be private/implementation details that can change between versions/may be unstable in runtime operation. If the database doesn't offer a public journaling API, then yes, sadly, sometimes the best or at least least-worst answer is to build a sub-par journal inside the database itself, with of course its own tradeoffs in performance.

cannadayr 8y ago

I solved this problem by wrapping sqldiff (included w/ sqlite src) as a custom git diff driver. https://github.com/cannadayr/git-sqlite https://github.com/cannadayr/git-sqlite

groue 8y ago

Related article: "Automatic Undo/Redo Using SQLite" https://www.sqlite.org/undoredo.html https://www.sqlite.org/undoredo.html

iffycan 8y ago
Thank you for posting this. A former version of my post included a reference to this because it's a great read, too.

radiospiel 8y ago

The sqlite sessions extension seems to be a good foundation for getting changelogs out of a sqlite database. It is sadly not too well known: https://www.sqlite.org/sessionintro.html https://www.sqlite.org/sessionintro.html

janemanos 8y ago

ArangoDB also has a first tool to listen to changes in the database and execute actions or queries. It is limited to a single node. Question is how scalable all of these tools are https://www.arangodb.com/2017/03/arangochair-tool-listening-changes-arangodb/ https://www.arangodb.com/2017/03/arangochair-tool-listening-...