5

We have defined a series of configurations, where, driven by a RESTful API, end-users can build up new revisions. Some of the components of the configuration can have more than one value; a revision involves multiple tables with one-to-many relationships.

Because the configuration is shipped off elsewhere, revisions are marked as deployed, and become immutable. Users have to create a new revision (which can be cloned from an existing one) if they want to make changes to a configuration. One revision per configuration can be marked as 'current'; this allows the users to switch between past revisions at will, or disable the configuration entirely by not picking any revision. The current revision is deployed, when marking a different revision as 'current' you replace the deployed config.

We already have everything in place to enforce immutability of deployed revisions; the deployed column is automatically transitioned to TRUE when you first use a revision as the current revision, and all further INSERT, UPDATE and DELETE operations concerning rows that match a deployed revision id in revision-related tables is blocked.

However, any value used for the name column in the public name table, must be unique across all the 'current' revisions across all current configurations. I'm trying to figure out the best strategy to enforce this.

If this was a plain one-to-many relationship from config to public names, this would be solved by using a unique constraint on the name column. This is, instead, a one-to-many-to-many pattern with revision acting as the bridge table, and the current_revision_id "collapses" the one-to-many-to-many to a virtual one-to-many relationship from config to public names.

Here is a simplified set of tables that illustrate our situation:

-- Configurations
CREATE TABLE config (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    current_revision_id INT
);

-- Have multiple revisions
CREATE TABLE revision (
    id INT PRIMARY KEY,
    config_id INT NOT NULL REFERENCES config(id),
    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
    description VARCHAR,
    foo INT NOT NULL,
    bar BOOLEAN NOT NULL,
    deployed BOOLEAN NOT NULL DEFAULT FALSE
);

-- A configuration has one _current_ revision
ALTER TABLE config
  ADD CONSTRAINT current_revision_id_fk
  FOREIGN KEY (current_revision_id)
  REFERENCES revision(id);
  
-- Revisions are automatically numbered in a view
CREATE VIEW numbered_revision AS (
    SELECT *,
    row_number() OVER (
        PARTITION BY config_id
        ORDER BY created_at, id
    ) AS number
    FROM revision
);

-- Configurations have multiple 'public names'
CREATE TABLE public_name (
    id INT PRIMARY KEY,
    revision_id INT NOT NULL REFERENCES revision(id),
    name VARCHAR(100),

    UNIQUE (revision_id, name)
);

The view only serves to provide revisions with gapless numbers per config (revisions are never deleted).

As an ERD diagram:

diagram of tables

Some sample data to illustrate the setup:

INSERT INTO config (id, name) VALUES
  (17, 'config_foo'),
  (42, 'config_bar');
  
INSERT INTO revision (id, config_id, created_at, description, foo, bar) VALUES
  (11, 17, '2021-05-29 09:07:18', 'Foo configuration, first draft', 81, TRUE),
  (19, 17, '2021-05-29 10:42:17', 'Foo configuration, second draft', 73, TRUE),
  (23, 42, '2021-05-29 09:36:52', 'Bar configuration, first draft', 118, FALSE);

INSERT INTO public_name (id, revision_id, name) VALUES
  -- public names for foo configuration, first draft
  (83, 11, 'some.name'),
  (84, 11, 'other.name'),
  -- public names for foo configuration, second draft
  (85, 19, 'revised.name'),
  (86, 19, 'other.name'),
  (87, 19, 'third.name'),
  -- public names for bar configuration, first draft;
  -- some of the names here are the same used by foo configurations
  (88, 23, 'some.name'),
  (89, 23, 'unique.name'),
  (90, 23, 'other.name');
  
-- Foo configuration has a current, published revision:
UPDATE config SET current_revision_id = 19 WHERE id = 17;
UPDATE revision SET deployed = TRUE WHERE id in (11, 19);

Here is a query showing the sample dataset:

SELECT
  c.name AS config,
  rev.number AS revision,
  rev.deployed,
  CASE WHEN c.current_revision_id = rev.id 
    THEN 'ACTIVE'
    ELSE ''
  END AS status,
  string_agg(p.name, ', ' ORDER BY p.name) AS names
FROM config c
JOIN numbered_revision AS rev ON c.id = rev.config_id
JOIN public_name p ON p.revision_id = rev.id
GROUP BY c.id, rev.id, rev.number, rev.deployed
ORDER BY c.id, rev.number;
config revision deployed status names
config_foo 1 t other.name, some.name
config_foo 2 t ACTIVE other.name, revised.name, third.name
config_bar 1 f other.name, some.name, unique.name

db<>fiddle here

In the above output table, the second row represents a "current" revision, made public deployed), and that row has been given exclusive access to the public names in the names column.

The third row represents a configuration with a draft revision. Any attempts to set it as current for config_bar should fail because the name other.name is already in use for config_foo, revision 2. If, in the future, config_foo were to create a new revision that doesn't include other.name, only then could config_bar revision 1 be made current.

We do pre-validate this constraint; the API runs some checks and blocks marking a configuration as current when pre-conditions are not met. Names in the public_name table are also constrained to be unique per revision (UNIQUE (revision_id, name)). Neither of these prevents a race condition, they just reduce the rate at which race conditions happen.

I was hoping a CONSTRAINT TRIGGER on config, firing on UPDATEs of the current_revision_id column, would be sufficient to enforce this constraint:

CREATE OR REPLACE FUNCTION unique_current_names() RETURNS trigger
   LANGUAGE plpgsql AS
$$BEGIN
   IF EXISTS (
     SELECT 1
     FROM public_name p
     WHERE
       p.revision_id = NEW.current_revision_id
     AND p.name IN (
       SELECT pp.name 
       FROM config AS pc
       JOIN public_name pp ON pp.revision_id = pc.current_revision_id
        AND pc.id != OLD.id
     )
   ) THEN
      RAISE EXCEPTION 'Public name is already published';
   END IF;
 
   RETURN NEW;
END;$$;
 
DROP TRIGGER IF EXISTS unique_current_names_trig ON config;
CREATE CONSTRAINT TRIGGER unique_current_names_trig
   AFTER UPDATE OF current_revision_id ON config
   DEFERRABLE INITIALLY DEFERRED
   FOR EACH ROW EXECUTE PROCEDURE unique_current_names();

(Note that the relationship between config and public_name is, in the general case, a many-to-many connection, but for the more specific current_revision_id case, it is a one-to-many connection, and you can use config.current_version_id = public_name.version_id to list the names directly.)

My concern is that, even though this trigger fires at the very end of a transaction, there is still the possibility of a race condition, wherein another connection also tries to make a revision current with conflicting public names.

OTOH, because all updates and inserts are the results of RESTFul API operations, there will never be a transaction that includes multiple operations (updates of public_name, and setting current_revision_id). Is that enough to prevent race conditions here, or are there corner cases I missed?

Another option might be to copy the public names of the current revision into a separate “published names” table (with a trigger; delete all old names, insert all new names), with a UNIQUE constraint on the name column there. Would that work better than the constraint trigger?

Note that we can’t use namespaces or other additions to the names (which are hostnames, on the public internet) to make them unique. The names must be unique entirely on their own, once deployed.

We are aware the design allows a configuration to reference a current revision_id that belongs to a different configuration. That’s a possibility we explicitly guard against at the application level, but a trigger could also handle that.

0
3

I did not follow the exact details of your data model, but a deferred constraint trigger is always subject to race conditions unless you operate with the SERIALIZABLE transaction isolation level.

The reason is that concurrent updates of config could cause the trigger function to run in parallel in two sessions, where they cannot see the effects of the other transaction, because no transaction has committed yet. Defining the trigger as INITIALLY DEFERRED narrows the window for the race condition, but it does not close it.

As an alternative to using SERIALIZABLE, you could modify your trigger function so that it takes locks that prevent it from running more than once at the same time. Transaction-level advisory locks come to mind as a simple way to do that.

4
  • Thanks for confirming the race condition; you may notice that my TRIGGER definition looks a lot like the one in that post (even more so in the first revision of my question)! :-) I'm still wondering if that race condition still applies if there are never going to be transactions that alter both the public_name and config tables at the same time (the RESTFul API doesn't let you). Jun 1 at 10:07
  • The trigger function selects from config. If there are concurrent updates on that table, each transaction cannot see the modifications from the other one. Of course, if you can exclude concurrent modifications, the problem vanishes. Jun 1 at 10:18
  • Right, I can indeed see a scenario now where this will indeed fail. Configs A and B both have a new revision claiming name “example”, the name has not been claimed before. Both update their config to make the revision current. The trigger fails to prevent this because neither transaction can see that another has now introduced a conflicting change. How would locking help here? The transaction that wins the race for the lock will have committed their change before the other can take the lock, so the change is now visible? Jun 1 at 12:21
  • About the lock: If you use the default read committed isolation level, PostgreSQL will read the current value as soon as it gets the lock, so it will see the committed value. Jun 1 at 12:36
3

The general idea in this solution is to add a little denormalization so the desired restrictions can all be enforced with regular foreign keys, check constraints, and filtered unique indexes.

I don't know Postgres well enough, so this is a SQL Server implementation.

The main features are:

  • Current revision moved from config to revisions
  • Current revision is zero if not current, otherwise matches revision id
  • Current revision denormalized to public names with cascade sync
  • Tricky uniqueness constraints implemented with filtered indexes

This design may or may not be acceptable, but perhaps it will at least provoke some thought.

DROP TABLE IF EXISTS dbo.PublicNames, dbo.Revisions, dbo.Configs;

Configurations

CREATE TABLE dbo.Configs (
    ConfigID integer NOT NULL,
    ConfigName varchar(100) NOT NULL,

    CONSTRAINT [PK dbo.Configs ConfigID]
        PRIMARY KEY CLUSTERED (ConfigID),

    -- Assuming configuration names are unique
    CONSTRAINT [UQ dbo.Configs ConfigName]
        UNIQUE NONCLUSTERED (ConfigName),
);

Revisions

CREATE TABLE dbo.Revisions (
    RevisionID integer NOT NULL,
    ConfigID integer NOT NULL,
    CurrentRevisionID integer NOT NULL 
        -- Revision is current if CurrentRevisionID = RevisionID
        -- Zero otherwise (see check constraints)
        DEFAULT 0,

    CreatedAt datetimeoffset NOT NULL 
        DEFAULT SYSDATETIMEOFFSET(),
    RevisionDescription varchar(200) NULL,
    Foo integer NOT NULL,
    Bar bit NOT NULL,

    -- Convenient computed column, persisted or not
    IsCurrent AS CONVERT(bit, 
        IIF(CurrentRevisionID = RevisionID, 'true', 'false')),
    IsDeployed bit NOT NULL 
        DEFAULT CONVERT(bit, 'false'),

    CONSTRAINT [PK dbo.Revisions RevisionID] 
        PRIMARY KEY CLUSTERED (RevisionID),

    CONSTRAINT [FK dbo.Revisions -> dbo.Configs ConfigID]
        FOREIGN KEY (ConfigID)
            REFERENCES dbo.Configs (ConfigID),

    CONSTRAINT [CK dbo.Revisions Valid CreatedAt]
        CHECK (CreatedAt <= SYSDATETIMEOFFSET()),

    -- RevisionID = 0 is a reserved value, must not be used
    CONSTRAINT [CK dbo.Revisions Valid RevisionID] 
        CHECK (RevisionID != 0),

    -- CurrentRevisionID must be zero or match RevisionID
    CONSTRAINT [CK dbo.Revisions Valid CurrentRevisionID] 
        CHECK (CurrentRevisionID IN (0, RevisionID)),

    -- A revision can only be deployed if it is current
    CONSTRAINT [CK dbo.Revisions Only Deployed If Current]
        CHECK (IsDeployed = 'false' OR 
            (CurrentRevisionID = RevisionID AND IsDeployed = 'true')),

    -- For denormalization via FK to dbo.PublicNames
    CONSTRAINT [UQ dbo.Revisions RevisionID, CurrentRevisionID] 
        UNIQUE NONCLUSTERED (RevisionID, CurrentRevisionID),

    -- Unique current revision per config
    INDEX [UQ dbo.Revisions One Current Revision Per Config]
        UNIQUE (ConfigID, IsCurrent)
        INCLUDE (CurrentRevisionID)
        WHERE CurrentRevisionID != 0
);

Public names

CREATE TABLE dbo.PublicNames (
    PublicNameID integer NOT NULL,
    RevisionID integer NOT NULL,
    CurrentRevisionID integer NOT NULL
        DEFAULT 0,
    PublicName varchar(100) NOT NULL,

    CONSTRAINT [PK dbo.PublicNames PublicNameID]
        PRIMARY KEY CLUSTERED (PublicNameID),

    CONSTRAINT [FK dbo.PublicNames -> dbo.Revisions RevisionID]
        FOREIGN KEY (RevisionID) 
            REFERENCES dbo.Revisions (RevisionID),

    -- Denormalized, kept in sync via cascade
    CONSTRAINT [FK dbo.PublicNames -> dbo.Revisions RevisionID, CurrentRevisionID]
        FOREIGN KEY (RevisionID, CurrentRevisionID)
        REFERENCES dbo.Revisions (RevisionID, CurrentRevisionID)
        ON UPDATE CASCADE,

    -- Public names unique within a revision
    CONSTRAINT [UQ dbo.PublicNames PublicName, RevisionID]
        UNIQUE NONCLUSTERED (PublicName, RevisionID),

    -- To support foreign key
    INDEX [IX dbo.PublicNames RevisionID, CurrentRevisionID]
        NONCLUSTERED (RevisionID, CurrentRevisionID),

    -- Public names unique across all current revisions
    INDEX [UQ dbo.PublicNames Unique Current Public Names]
        UNIQUE NONCLUSTERED (PublicName)
        INCLUDE (CurrentRevisionID)
        WHERE CurrentRevisionID != 0
);

Data

Sample data and state as provided in the question:

INSERT INTO dbo.Configs (ConfigID, ConfigName) VALUES
    (17, 'config_foo'),
    (42, 'config_bar');
  
INSERT INTO dbo.Revisions (RevisionID, ConfigID, CreatedAt, RevisionDescription, Foo, Bar) VALUES
    (11, 17, '2021-05-29 09:07:18', 'Foo configuration, first draft', 81, 'true'),
    (19, 17, '2021-05-29 10:42:17', 'Foo configuration, second draft', 73, 'true'),
    (23, 42, '2021-05-29 09:36:52', 'Bar configuration, first draft', 118, 'false');

INSERT INTO dbo.PublicNames (PublicNameID, RevisionID, PublicName) VALUES
  -- public names for foo configuration, first draft
  (83, 11, 'some.name'),
  (84, 11, 'other.name'),
  -- public names for foo configuration, second draft
  (85, 19, 'revised.name'),
  (86, 19, 'other.name'),
  (87, 19, 'third.name'),
  -- public names for bar configuration, first draft;
  -- some of the names here are the same used by foo configurations
  (88, 23, 'some.name'),
  (89, 23, 'unique.name'),
  (90, 23, 'other.name');
  
-- Foo configuration has a current, published revision:
UPDATE dbo.Revisions SET 
    CurrentRevisionID = 19, 
    IsDeployed = 'true' 
WHERE ConfigID = 17 
AND RevisionID = 19;

Query

The status query:

WITH 
    NumberedRevisions AS
    (
        SELECT 
            R.*,
            Revision =
                ROW_NUMBER() OVER (
                    PARTITION BY R.ConfigID
                    ORDER BY R.CreatedAt, R.RevisionID)
        FROM dbo.Revisions AS R
    )
SELECT
    C.ConfigName,
    R.Revision,
    R.IsDeployed,
    R.IsCurrent,
    Names =
        STRING_AGG(P.PublicName, ',')
            WITHIN GROUP (
                ORDER BY P.PublicName)
FROM dbo.Configs AS C
JOIN NumberedRevisions AS R
    ON R.ConfigID = C.ConfigID
JOIN dbo.PublicNames AS P
    ON P.RevisionID = R.RevisionID
GROUP BY
    C.ConfigID,
    C.ConfigName, 
    R.Revision, 
    R.IsDeployed, 
    R.IsCurrent
ORDER BY
    C.ConfigID,
    Revision;
ConfigName Revision IsDeployed IsCurrent Names
config_foo 1 0 0 other.name,some.name
config_foo 2 1 1 other.name,revised.name,third.name
config_bar 1 0 0 other.name,some.name,unique.name

Test

Attempt to set the third row current:

UPDATE dbo.Revisions SET
    CurrentRevisionID = 23
WHERE ConfigID = 42
AND RevisionID = 23;

Error:

Msg 2601, Level 14, State 1
Cannot insert duplicate key row in object 'dbo.PublicNames' with unique index 
'UQ dbo.PublicNames Unique Current Public Names'. 
The duplicate key value is (other.name).
The statement has been terminated.

db<>fiddle demo


The above is a fairly standard technique to allow engine-enforced denormalization, so we can use constraints and filtered indexes to enforce data integrity—regardless of concurrency or other concerns (product bugs aside). See Denormalizing to enforce business rules by Alexander Kuznetsov for more.

There will usually be application-side impacts, but these often turn out to be surprisingly manageable in practice. In SQL Server, that is often done by abstracting any additional implementation complexity into a module (stored procedure or function, usually). That preserves a clean and consistent API.

One can't beat the good night's sleep you get, safe in the knowledge the database is consistent at all times.

2
  • That's an interesting approach; so instead of a trigger, you make use of ON UPDATE CASCADE copying across the CurrentRevisionId value to the public name rows. Is that why the CurrentRevisionId column is using 0 as the sentinel value instead of NULL? The downside of course is that now you have to update two rows when promoting a revision to the current one, complicating application logic a little. Jun 2 at 11:22
  • And interestingly enough, our application logic already includes an active computed value to indicate if a revision is currently the published revision for the config. Jun 2 at 11:24
1

Using a constraint trigger will not prevent a race condition, as confirmed by Laurenz Albe, unless we switch to the transaction isolation level to SERIALIZABLE. That would complicate our application logic, as we'd have to retry commits.

Instead of using a constraint trigger, we are now simply copying the published public names into a new table using an ON INSERT OR UPDATE trigger. The unique constraint is then enforced on that table, and so not subject to the same transaction isolation issues. If two transactions were to attempt to promote a revision with a conflicting name to being public, one or the other transaction will fail on the unique constraint.

This is what the published_public_name table, and the trigger on config looks like:

CREATE TABLE published_public_name (
  id INT PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,
  config_id INT NOT NULL REFERENCES config(id) ON DELETE CASCADE,
  name VARCHAR(100) NOT NULL UNIQUE
);

CREATE OR REPLACE FUNCTION copy_published_public_names()
RETURNS TRIGGER
AS $BODY$
BEGIN
  -- OLD / NEW is a row in the config table
  -- Trigger is called for both updates and inserts
  IF TG_OP = 'UPDATE' THEN
    IF OLD.current_revision_id IS NOT DISTINCT FROM NEW.current_revision_id THEN
      -- Nothing changed
      RETURN NEW;
    END IF;

    IF OLD.current_revision_id IS NOT NULL THEN
      DELETE FROM published_public_name
      WHERE id IN (
        SELECT ppn.id
        FROM published_public_name ppn
        WHERE ppn.config_id = OLD.id
        ORDER BY ppn.name
      );
    END IF;
  END IF;

  IF NEW.current_revision_id IS NOT NULL THEN
    INSERT INTO published_public_name(config_id, name)
    SELECT NEW.id, pn.name
    FROM public_name pn
    WHERE pn.revision_id = NEW.current_revision_id
    ORDER BY pn.name;
  END IF;
  
  RETURN NEW;
END
$BODY$
LANGUAGE plpgsql;

CREATE TRIGGER copy_published_public_names_trigger
BEFORE INSERT OR UPDATE OF current_revision_id ON config
FOR EACH ROW
EXECUTE FUNCTION copy_published_public_names();

This handles INSERT and UPDATE. DELETES are handled by ON DELETE CASCADE on the published_public_name.config_id foreign key.

I included an ORDER BY clause in the DELETE and INSERT statements in the trigger to avoid a potential deadlock if more than one name is involved. I'm not 100% certain that the DELETE ordering is required, but it does no harm if not.

When trying to publish a revision with a name that's already used by another config, the statement fails with:

ERROR:  duplicate key value violates unique constraint "published_public_name_name_key"
DETAIL:  Key (name)=(other.name) already exists.
CONTEXT:  SQL statement "INSERT INTO published_public_name(config_id, name)
        SELECT NEW.id, pn.name
        FROM public_name pn
        WHERE pn.revision_id = NEW.current_revision_id"
PL/pgSQL function copy_published_public_names() line 18 at SQL statement

See my new db<>fiddle revision.

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