schemadiff: Vitess In-memory Schema Diffing, Normalization, Validation and Manipulation

schemadiff: Vitess In-memory Schema Diffing, Normalization, Validation and Manipulation

Introducing schemadiff, an internal library in Vitess that has been one of its best-kept secrets until now. At its core, schemadiff is a declarative, programmatic library that can produce a diff in SQL format of two entities: tables, views, or full blown database schemas. But it then goes beyond that to normalize, validate, export, and even apply schema changes, all declaratively and without having to use a MySQL server. Let's dive in to understand its functionality and capabilities.

Some tech specs #

schemadiff supports MySQL 8.0 dialect and functionality. It is completely in-memory and does not use any MySQL server or MySQL code. It applies MySQL-compatible logic and rules to validate and apply schema changes. It uses a declarative approach but also works with imperative commands.

Diff objectives #

schemadiff, as its name suggests, began as a diffing library. The objectives were:

  1. Given two table definitions, what schema changes (DDLs) would we need to apply on the first, so it looks like the second?
  2. Given two schemas (aka databases), that include tables and views, what schema changes (DDLs) would we need to apply on the first, so it looks like the second?

As a use case for (1), consider declarative Vitess migrations. Consider that you might want to tell your database: "Here's a table, please make it look like so". It's the database's job to determine whether this table exists in the first place or not. If not, then it must be created. If it exists, and already looks exactly like you want it to, great, that's a no-op. But what if the current table looks somewhat different? What changes need to be applied on the existing table so as to make it look like your desired table? Specifically, what ALTER TABLE statement(s) (there are some rare scenarios where we might need to invoke more than one) do we need to invoke?

Case (2) is best explained with PlanetScale branching. You have a production schema, and you have a local copy of that schema. On your local copy, you make changes over time, and finally wish to apply those changes in production. Did you track those changes? Some ORMs will do a good job at that. But you might not have used an ORM, or the ORM might not be feature complete. What tables do you need to CREATE? Which views should be DROPped? And what needs to be ALTERed? Is there a specific order of operations?

There are two approaches to analyzing the diff of tables or schemas. One approach is to use a running MySQL server. Deploy the two schemas on that server, then investigate the INFORMATION_SCHEMA metadata tables and construct a model for the schema. INFORMATION_SCHEMA offers a formal breakdown such as the columns found in each table, what data type a column has, what default does it have, if any; which indexes are unique, which columns do they cover; what foreign key constraints on a table point to which parent table, what columns are covered; etc.

This approach has two advantages:

  1. If you're able to create a table in MySQL, that means it's valid. By the time you introspect INFORMATION_SCHEMA on a table, validity is a given. You may safely assume, for example, that keys only cover existing columns.
  2. INFORMATION_SCHEMA formalizes the majority of information. The data type is well defined. A column precision is an integer value. The text collation is a well known value.

However, there are disadvantages, as well:

  • Complex expressions are left, well, complex. A CREATE VIEW ... statement is really just a long SQL text. Which tables or views are referenced in our VIEW definition? Which columns?
  • Not everything is normalized and formalized. We know int(10) and int(11) are really the same. But as MySQL goes, the column type is different. Contrast this with varchar(10) and varchar(11), where the type is truly different. It still takes some higher level logic to understand what is a real diff and what isn't.
  • MySQL allows schema inconsistencies. It is possible, in MySQL, to have an "orphaned" VIEW, where the tables/views on which it relies, do not exist. Reading something from MySQL doesn't mean it's really valid.
  • Last, and most impactful of all, is the operational overhead. To diff two schemas, you need to deploy two schemas, then read back the metadata for all the tables, views, indexes, constraints, etc. You need to deploy a MySQL server, likely in a non-production environment. You will need to start the server, deploy the changes, read back, shutdown the server. This is a heavyweight operation. And if you then want to validate your diff, you probably want to deploy it on the running server, then re-read the result, compare it with what you were expecting to find. This is even more heavyweight. And, if you want to compute the order in which the changes must take place (some scenarios dictate a specific order, a topic for a future post), then you'd need to solicit confirmation from MySQL by applying changes on the server, iteratively.

The second is the in-memory, programmatic approach. Instead of relying on INFORMATION_SCHEMA, we rely on the SQL of the schema, i.e. on the CREATE TABLE and CREATE VIEW statements themselves. This poses a few challenges of its own:

  • First and foremost, you must be able to parse and analyze all statements. This includes a CREATE TABLE that has a CHECK CONSTRAINT with a complex expression, or a sub-partitioning scheme using functions over columns. You also need the ability to fully parse a CREATE VIEW statement, a complex GENERATED column expression, etc.
  • You must be able to accommodate different equivalent definitions. For example, create table t (id int primary key) is equivalent to create table t(id int, primary key (id)). Even MySQL's own SHOW CREATE TABLE output may present different results depending on when/where the table was created.
  • Not having a MySQL server to validate correctness of the initial schema and the generated diffs means that the application must implement that logic.

schemadiff uses the programmatic approach. Let's look at some sample code first and then we'll move on to discuss its internals.

Quick diff examples #

By way of simple illustration, we create and diff two schemas, each with a single table. First schema:

schema1, err := NewSchemaFromSQL("create table t (id int, name varchar(64), primary key (id))")
if err == nil {
	`id` int,
	`name` varchar(64),

In the second schema, our table is slightly modified:

schema2, err := NewSchemaFromSQL("create table t (id bigint, name varchar(64), key name_idx(name(16)), primary key (id))")
if err == nil {
	`id` bigint,
	`name` varchar(64),
	KEY `name_idx` (`name`(16)),

We now programmatically diff the two schemas (this is actually the long path to doing so):

hints := &DiffHints{}
diff, err := schema1.SchemaDiff(schema2, hints)
// Handle error
diffs, err := diff.OrderedDiffs()
// Handle error
for _, diff := range diffs {

And this is the resulting diff:

ALTER TABLE `t` MODIFY COLUMN `id` bigint, ADD KEY `name_idx` (`name`(16))

There are multiple ways of generating the diff. Here's a quick shortcut to achieving the same:

diffs, err := DiffSchemasSQL("create ...", "create ...", hints)

The main thing to note in the above examples is that everything takes place purely within go space, and MySQL is not involved. schemadiff is purely programmatic, and makes heavy use of Vitess' sqlparser library.

sqlparser #

Vitess is a sharding and management framework running on top of MySQL, which masquerades as a MySQL server to route queries to relevant shards. It thus obviously must be able to parse MySQL's SQL dialect. sqlparser is the Vitess library that does so.

sqlparser utilizes a classic yacc file to parse SQL into an Abstract Syntax Tree (AST), with golang structs generated and populated by the parser. For example, a SQL CREATE TABLE statement is parsed into a CreateTable instance:

CreateTable struct {
	Temp        bool
	Table       TableName
	IfNotExists bool
	TableSpec   *TableSpec
	OptLike     *OptLike
	Comments    *ParsedComments
	FullyParsed bool

And here is the breakdown of TableSpec:

type TableSpec struct {
	Columns         []*ColumnDefinition
	Indexes         []*IndexDefinition
	Constraints     []*ConstraintDefinition
	Options         TableOptions
	PartitionOption *PartitionOption

You can already see how AST helps us in analyzing a table's definition. As a very simple illustration, imagine we have two tables we want to diff. Say we want to determine whether the two have a different set of columns (let's ignore ordering for now). How would we do that?

We can programmatically iterate over range table1.TableSpec.Columns in each of the tables. We can do a full drill down of all the details in a ColumnDefinition. Or, we can take a shortcut. schemadiff uses an optimistic approach: most of the schema is likely to be identical. It first attempts to compare components as a whole. If the components are not identical as a whole, we can proceed to drill down.

How do you "compare a component as a whole"? sqlparser not only parses SQL, it also exports AST as SQL. We can, for example, run:

  s1 := sqlparser.CanonicalString(table1.TableSpec.Columns[i])
  s2 := sqlparser.CanonicalString(table2.TableSpec.Columns[i])

We can thus compare any two nodes. If they export into identical strings, then they are equal and there is no need to drill down. sqlparser's AST also comes with an auto-generated set of deep-equals comparison and deep-copy functionality.

Continuing our simple illustration, if table2 has a column called info, which we cannot find in table1, then our table diff, i.e. our ALTER TABLE statement, should include an ADD COLUMN <column definition> statement. What's the content of the column definition? It is trivially the CanonicalString() of the extra column object.

sqlparser is devoid of semantic context. It merely deals with a programmatic reflection of SQL. It does not know if a certain table exists, or if a foreign key references valid columns. As long as the syntax is valid, it is satisfied.

Semantics #

The AST's sole purpose is to faithfully represent a SQL query/command. But as a by-product, it can also serve as the base to a semantic analysis of the schema. Consider the following table definition:

CREATE TABLE `invalid` (
	`id` bigint,
	`title` varchar(64),
	`title` tinytext,
	PRIMARY KEY (`val`)

The above table is syntactically valid, but semantically invalid. There are two columns that go by the same name (title), and an index that covers a non-existent column (val). These are but two out of many possible errors. A SHOW CREATE TABLE would never produce such an output, of course. But we can't assume the output comes from MySQL. If we don't validate the input, and attempt to produce a diff, the results could be unexpected.

Validation #

The basic premise of validation is that all users want their schemas to be valid. There's no point in using invalid schemas, because, at the end of they day, you want to be able to deploy those schemas on real MySQL servers.

To that effect, schemadiff enforce validation upon loading a new schema. As we see later on, it also enforces validation upon applying changes. If you try to load an invalid schema as in the above, schemadiff returns an informative error. You can't have two columns of same name. Your keys may only cover existing columns. A collation name must be recognized. A GENERATED column must refer to existing columns. A FOREIGN KEY constraint must reference existing columns in existing tables, and foreign key columns mast have matching data types, etc. Circular FOREIGN KEY dependencies are not allowed.

Even more interesting is view validation. schemadiff offers a complete validation over view definitions: all referenced tables/views must exist. Circular dependency is not allowed. Referenced columns are validated to exist. Unqualified column names are ensured not to be ambiguous.

Diff hints #

There are nuances. What do you do about different AUTO_INCREMENT values? These are not strictly part of the schema, but may or may not cause impact to production if you apply or not apply them. How do you deal with renamed columns? What about constraint names, which, per ANSI SQL, must be unique across the schema? Do you mind if key definitions are in different order?

DiffHints includes many controls that affect the diffing logic and output.

Normalization #

Consider this table definition:

create table what_can_be_normalized (
	id int primary key,
	i int(12) default null,
	v varchar(32) collate utf8mb4_0900_ai_ci
) default charset=utf8mb4 collate=utf8mb4_0900_ai_ci

There's a few things to normalize here. Inputs can come in different shapes and sizes, and still mean the same thing. Even MySQL itself presents different output for textual columns based on which version the table was created in. In the above table definition, we can normalize the following:

  • The canonical way to declare a primary key is as an index definition, not as part of the column definition.
  • int(12) is just an int. 12 does not matter. Integer precision is in fact being deprecated in MySQL. Interestingly, some ORMs do have special treatment for int(1), as an indication to a boolean value. schemadiff accommodates that.
  • It looks like i is nullable (because it does not say not null). Which means its default value is null even if we don't explicitly say that.
  • v's collation, and thereby character set, agree with the table's collation. It can be removed.
  • And, of course, the snippet uses unqualified names and lower case SQL syntax.

A normalized version of the above looks like:

CREATE TABLE `what_can_be_normalized` (
	`id` int,
	`i` int,
	`v` varchar(32),
) CHARSET utf8mb4,
  COLLATE utf8mb4_0900_ai_ci;

schemadiff chooses to normalize into the shortest form possible, often shorter than MySQL's own SHOW CREATE TABLE output. Normalization ensures we converge onto a single, canonical, representation of a table, view, or schema.

Beyond diff #

Up till now, we saw that we can load a schema into schemadiff. schemadiff would normalize the schema, validate it, and throw an error if it's invalid. We can load a 2nd schema, and if all goes well we can diff the two, and grab the diffs.

What then? Do we trust those diffs? Will they result in the correct schema? Will they result in a valid schema?

It looks like to validate those diffs we'd need to run a MySQL server, deploy the 1st schema, apply those diffs (CREATE, ALTER, DROP statements), then re-read the schema and compare it with the one we aimed for. But, this brings back the dependency on a MySQL server, something we wanted to avoid in the first place.

Unless, we can ask schemadiff to programmatically apply those diffs and validate the result!

Applying diffs #

Consider this simplified code, or see snippet from diff_test.go:

schema1, err := NewSchemaFromSQL(sql1)
// Handle error

schema2, err := NewSchemaFromSQL(sql2)
// Handle error

diff, err := schema1.SchemaDiff(schema2, hints)
// Handle error
diffs, err := diff.OrderedDiffs()
// Handle error

applied, err := schema1.Apply(diffs)
require.NoError(t, err)

appliedDiff, err := schema2.SchemaDiff(applied, hints)
require.NoError(t, err)
assert.True(t, appliedDiff.Empty())
assert.Equal(t, schema2.ToQueries(), applied.ToQueries())

schemadiff lets us take those diffs and Apply() them onto a schema (schema1), resulting with a new schema. We expect that result to be equal in content to the expected (schema2) schema. We also expect schemadiff itself to agree the two are identical.

Apply() makes an in-memory schema change. You may CREATE TABLE or ALTER TABLE or DROP VIEW, all programmatically, and all using AST structures such as CreateTable, AlterTable, or DropView, etc.

Apply() validates the requested changes on the fly, as well as on the resulting schema. It adheres to MySQL rules. For example:

  • If the diff has a AddColumn AST struct, verify upfront that no column exists by same name.
  • If there's an AddKey, validate that the columns specified by the key do in fact exist.
  • If there's an AddKey and no index name specified, generate one, compatible with MySQL naming.
  • Conversely, if there's a DropColumn, and some keys exist that actually cover that column, remove the column from those keys; any key that is left without any covered columns is dropped (this is the standard MySQL behavior).
  • You shouldn't be able to DropColumn if there's a FOREIGN KEY referencing that column, though.
  • The list goes on.

As we can see, there are a lot of validations involved. Some of them actually depend on each other! Say we drop a column as well as a FOREIGN KEY that references that column. MySQL-wise this is a single ALTER TABLE operation. But, programmatically, schemadiff needs to first remove the FOREIGN KEY, and then remove the column. The reverse order is invalid. schemadiff computes the correct order of operations for a table.

But then, it also computes the correct order of operations in the grand context of the schema. Consider this diff:

create view v1 as select * from v2;
create view v2 as select * from t;

Alphabetically, v1 comes before v2. But, to apply these two diffs, we must first create v2, then v1. The reverse order is invalid. This time it's invalid not only programmatically in schemadiff, but also when applied to MySQL itself.

schemadiff maintains the hierarchical ordering of all tables and views, based on FOREIGN KEY and view definitions. There's a specific order for CREATE statements, and a specific (reverse) order for DROP statements. There may actually be conflicts between the diffs, but that's a topic for a different blog post.

Or, consider the flow in this e2e test. We hook onto vitess's pre-existing Online DDL tests, a suite that includes a multitude of scenarios. The suite already has a story: a table, a change, an expected result. In our schemadiff test we shuffle the logic: we have an original table and and expected table. We evaluate the diff. We apply the diff onto the original table. We expect the result to be identical to the expected table!

Because this runs in GitHub CI, we take advantage of a running MySQL server. We perform the test in-memory, and then also on MySQL itself, and so we have an authoritative validation for the Apply() functionality. For fun and glory, we also generate the reverse diff. This test cross validates the programmatic approach, the MySQL schema, and actual MySQL schema changes, and expects full compliance between all.

Even beyond #

The declarative approach is challenging. It requires reverse-engineering of a hidden sequence of changes. Sometimes these changes are inter-dependent. We will discuss this further in a future post.

Note that schemadiff is an internal library, and as such, its interface is subject to change. As part of the Vitess codebase, schemadiff code is open source, licensed under Apache 2.0.