Re: Stray changes detected only sometimes with revision --autogenerate
Hi Mike, I adapted our setup to the new cookbook recipe and it basically fixed both problems! I don't need to maintain my patches anymore (thanks to setting `connection.dialect.default_schema_name`, this did the trick!) and the detected stray changes are also gone. I did some investigation with increased log level as you suggested and came to the conclusion that it must have been a bad idea to manipulate the `conn_table_names` object in-place. But since that's gone now, everything is working as expected \o/ Thank you so much for your help! Cheers, Daniel On 29.09.20 19:04, Mike Bayer wrote: Hi, so I added a quick recipe to the site just now just so that the "set search path" idea is documented to some extent, that is at https://alembic.sqlalchemy.org/en/latest/cookbook.html#rudimental-schema-level-multi-tenancy-for-postgresql-databases . Re: autogenerate, if you have many schemas that all contain an exact copy of the same schema, then autogenerate implies you'd be running this exactly once for just one of the schemas, or a "master" schema, such that you generate the .py files that will then be applicable to all the schemas. The recipe above discusses this, and in particular it works in a completely schema agnostic fashion, you would set include_schemas=False in the config and allow the PostgreSQL search_path to handle all movement between schemas. then for the actual migration "upgrade" run, you would run those files against each schema in sequence, again making use of search_path in order to select each schema. as far as how alembic is looking at current tables you would need to turn on SQL logging, using the "debug" level, such as in alembic.ini (if you're using that): [logger_sqlalchemy] level = DEBUG handlers = qualname = sqlalchemy.engine this will emit a lot of queries and result sets. You'd have to capture that logging and then grep through for the "nonexistent" schema - it will be present in a result set that Alembic is capturing, most fundamentally the one it uses to get all the schema names. On Tue, Sep 29, 2020, at 4:01 PM, Daniel Krebs wrote: Hi Mike, thanks a bunch for the quick response! >> This is supported by SQLAlchemy more robustly using the >> schema_translate_map feature, see >> https://docs.sqlalchemy.org/en/13/core/connections.html#schema-translating >> . this feature allows schema names to render explicitly without the >> need to manipulate search_path. > > this feature may not work completely with Alembic however, I'm > evaluating that now. > > still doesnt shed any light on your sporadic problem, however. It's been some time already since I've implemented this, so can't recall the exact reason why I went down that road but I know for sure that I was trying to make it work with schema_translate_map but didn't succeed. I would have very much preferred a solution without patching alembic it seemed to my last resort at the time. I'd be more than happy to find a solution here :) > However, when using that approach you'd normally be running autogenerate from just one schema, since you are looking to generate migrations just once and then run them on every tenant. > > so it's not clear why autogenerate is being run on every tenant explicitly - you'd have just one "model" schema that's the one where you actually run autogenerate upon. there would be no need to consider other schemas and include_schemas would be set to False. Indeed that sounds like a good idea. So you'd suggest to keep one schema just for autogeneration purposes and then apply the migrations to all customer schemas? Is that possible from within alembic or would you wrap some tooling around alembic in order to apply migrations one-by-one to every schema? > As far as being able to filter out schemas across many, the current approach is the include_object hook, which is insufficient for many schemas as it does not block the reflection of all the tables. a new hook include_name is being added in the coming weeks that allows for pre-emptive inclusion or exclusion of specific schema names. I tried the include_object hook back then, but I was facing reflection issues if I remember correctly. We're using TimescaleDB [1] in the same database which adds some schemas of its own that alembic doesn't seem to be able to handle. > I don't have much insight onto this other than looking at concurrent activities on the database. "sometimes" usually indicates a race condition of some sort, and the "autogenerate" process is strictly one of reading data. Indeed I'm suspecting the same thing. However, I'm not the most knowledgable DBA nor do I have an understanding of how alembic actually compares the current state of the DB with the SA models. Could you maybe provide a more concrete point what and where to look for? Than
Re: Stray changes detected only sometimes with revision --autogenerate
Hi Mike, thanks a bunch for the quick response! This is supported by SQLAlchemy more robustly using the schema_translate_map feature, see https://docs.sqlalchemy.org/en/13/core/connections.html#schema-translating . this feature allows schema names to render explicitly without the need to manipulate search_path. this feature may not work completely with Alembic however, I'm evaluating that now. still doesnt shed any light on your sporadic problem, however. It's been some time already since I've implemented this, so can't recall the exact reason why I went down that road but I know for sure that I was trying to make it work with schema_translate_map but didn't succeed. I would have very much preferred a solution without patching alembic it seemed to my last resort at the time. I'd be more than happy to find a solution here :) > However, when using that approach you'd normally be running autogenerate from just one schema, since you are looking to generate migrations just once and then run them on every tenant. > > so it's not clear why autogenerate is being run on every tenant explicitly - you'd have just one "model" schema that's the one where you actually run autogenerate upon. there would be no need to consider other schemas and include_schemas would be set to False. Indeed that sounds like a good idea. So you'd suggest to keep one schema just for autogeneration purposes and then apply the migrations to all customer schemas? Is that possible from within alembic or would you wrap some tooling around alembic in order to apply migrations one-by-one to every schema? > As far as being able to filter out schemas across many, the current approach is the include_object hook, which is insufficient for many schemas as it does not block the reflection of all the tables. a new hook include_name is being added in the coming weeks that allows for pre-emptive inclusion or exclusion of specific schema names. I tried the include_object hook back then, but I was facing reflection issues if I remember correctly. We're using TimescaleDB [1] in the same database which adds some schemas of its own that alembic doesn't seem to be able to handle. > I don't have much insight onto this other than looking at concurrent activities on the database. "sometimes" usually indicates a race condition of some sort, and the "autogenerate" process is strictly one of reading data. Indeed I'm suspecting the same thing. However, I'm not the most knowledgable DBA nor do I have an understanding of how alembic actually compares the current state of the DB with the SA models. Could you maybe provide a more concrete point what and where to look for? Thank you for taking the time to help! Cheers, Daniel [1] https://www.timescale.com/ -- You received this message because you are subscribed to the Google Groups "sqlalchemy-alembic" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy-alembic+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy-alembic/fc2bc5e4-274b-a9e7-0566-3b745c4e5fc5%40enlyze.com.
Stray changes detected only sometimes with revision --autogenerate
Hi, we're having rather strange problems with Alembic 1.4.2 and Postgres 12, detecting stray changes *sometimes* but also sometimes not. I already dug through the code but I increasingly get the feel that this is rooted somewhere in the interaction between alembic and Postgres. But let me explain our setup first. We maintain a set of SQLAlchemy model definitions that we evolve over time and use alembic to migrate the database accordingly. For every customer, we add a new Postgres schema to the database identified by a corresponding UUID (e.g. a3d74dcc-1634-33a5-ff74-235f3a7c6322). See [1] for our (stripped) env.py. Since we want all customers to use the same DDL, we want common migrations that disregard the schema name altogether. It seems that this is not a supported use-case of alembic, so I hacked around it such that alembic only sees schema-less objects and I run SET search_path TO "uuid" before each migration (see env.py [1]). The patch should be rather straight-forward and can be found at [2]. Now the issue that we're facing is, that *sometimes* autogenerate detects changes in one or two customer schemas that are not real [3]. Deleting the migration and creating a new one often doesn't detect the changes anymore or for a different customer/schema. The tables that are incorrectly found to have changed also change over time. My current workaround is to autogenerate migrations until alembic "does the right thing". I know that patching alembic is not the best base upon which to ask for support, but to be honest I am running out of theories what is going wrong here. Hope someone can help or point me into the right direction :) Cheers, Daniel [1] https://gist.github.com/daniel-k/114aa2ac846c02e437b8d86ab89d21ac#file-env-py [2] https://gist.github.com/daniel-k/114aa2ac846c02e437b8d86ab89d21ac#file-alembic_patch-diff [2] https://gist.github.com/daniel-k/114aa2ac846c02e437b8d86ab89d21ac#file-log-txt -- You received this message because you are subscribed to the Google Groups "sqlalchemy-alembic" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy-alembic+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy-alembic/479fee75-e006-4c07-9ab3-149250205521n%40googlegroups.com.
