eventsourcingdb

The official Go client SDK for EventSourcingDB – a purpose-built database for event sourcing.

EventSourcingDB enables you to build and operate event-driven applications with native support for writing, reading, and observing events. This client SDK provides convenient access to its capabilities in Go.

For more information on EventSourcingDB, see its official documentation.

This client SDK includes support for Testcontainers to spin up EventSourcingDB instances in integration tests. For details, see Using Testcontainers.

Getting Started

Install the client SDK:

go get github.com/thenativeweb/eventsourcingdb-client-golang

Import the package and create an instance by providing the URL of your EventSourcingDB instance and the API token to use:

import (
  "net/url"

  "github.com/thenativeweb/eventsourcingdb-client-golang/eventsourcingdb"
)

// ...

baseURL, err := url.Parse("http://localhost:3000")
if err != nil {
  // ...
}

apiToken := "secret"

client, err := eventsourcingdb.NewClient(baseURL, apiToken)
if err != nil {
  // ...
}

Then call the Ping function to check whether the instance is reachable. If it is not, the function will return an error:

err := client.Ping()
if err != nil {
  // ...
}

Note that Ping does not require authentication, so the call may succeed even if the API token is invalid.

If you want to verify the API token, call VerifyAPIToken. If the token is invalid, the function will return an error:

err := client.VerifyAPIToken()
if err != nil {
  // ...
}

Writing Events

Call the WriteEvents function and hand over a slice with one or more events. You do not have to provide all event fields – some are automatically added by the server.

Specify Source, Subject, Type, and Data according to the CloudEvents format.

For Data, you may provide any struct with public fields. It is recommended to have JSON annotations on this struct to control how the struct gets serialized.

The function returns the written events, including the fields added by the server:

type BookAcquired struct {
  Title  string `json:"title"`
  Author string `json:"author"`
  ISBN   string `json:"isbn"`
}

event := eventsourcingdb.EventCandidate{
  Source:  "https://library.eventsourcingdb.io",
  Subject: "/books/42",
  Type:    "io.eventsourcingdb.library.book-acquired",
  Data: BookAcquired{
    Title:  "2001 – A Space Odyssey",
    Author: "Arthur C. Clarke",
    ISBN:   "978-0756906788",
  },
}

writtenEvents, err := client.WriteEvents(
  []eventsourcingdb.EventCandidate{
    event,
  },
  nil,
)

Using the isSubjectPristine precondition

If you only want to write events in case a subject (such as /books/42) does not yet have any events, use the NewIsSubjectPristinePrecondition function to create a precondition and pass it in a slice as the second argument:

writtenEvents, err := client.WriteEvents(
  []eventsourcingdb.EventCandidate{
    // ...
  },
  []eventsourcingdb.Precondition{
    eventsourcingdb.NewIsSubjectPristinePrecondition("/books/42"),
  },
)

Using the isSubjectOnEventId precondition

If you only want to write events in case the last event of a subject (such as /books/42) has a specific ID (e.g., 0), use the NewIsSubjectOnEventIDPrecondition function to create a precondition and pass it in a slice as the second argument:

writtenEvents, err := client.WriteEvents(
  []eventsourcingdb.EventCandidate{
    // ...
  },
  []eventsourcingdb.Precondition{
    eventsourcingdb.NewIsSubjectOnEventIDPrecondition("/books/42", "0"),
  },
)

Note that according to the CloudEvents standard, event IDs must be of type string.

Reading Events

To read all events of a subject, call the ReadEvents function with a context, the subject and an options object. Set the Recursive option to false. This ensures that only events of the given subject are returned, not events of nested subjects.

The function returns an iterator, which you can use e.g. inside a for range loop:

for event, err := range client.ReadEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ReadEventsOptions{
    Recursive: false,
  },
) {
  // ...
}

Reading From Subjects Recursively

If you want to read not only all the events of a subject, but also the events of all nested subjects, set the Recursive option to true:

for event, err := range client.ReadEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ReadEventsOptions{
    Recursive: true,
  },
) {
  // ...
}

This also allows you to read all events ever written. To do so, provide / as the subject and set Recursive to true, since all subjects are nested under the root subject.

Reading in Anti-Chronological Order

By default, events are read in chronological order. To read in anti-chronological order, provide the Order option and set it using the OrderAntichronological function:

for event, err := range client.ReadEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ReadEventsOptions{
    Recursive: false,
    Order:     eventsourcingdb.OrderAntichronological(),
  },
) {
  // ...
}

Note that you can also use the OrderChronological function to explicitly enforce the default order.

Specifying Bounds

Sometimes you do not want to read all events, but only a range of events. For that, you can specify the LowerBound and UpperBound options – either one of them or even both at the same time.

Specify the ID and whether to include or exclude it, for both the lower and upper bound:

for event, err := range client.ReadEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ReadEventsOptions{
    Recursive:  false,
    LowerBound: &eventsourcingdb.Bound{
      ID:   "100",
      Type: eventsourcingdb.BoundTypeInclusive,
    },
    UpperBound: &eventsourcingdb.Bound{
      ID:   "200",
      Type: eventsourcingdb.BoundTypeExclusive,
    },
  },
) {
  // ...
}

Starting From the Latest Event of a Given Type

To read starting from the latest event of a given type, provide the FromLatestEvent option and specify the subject, the type, and how to proceed if no such event exists.

Possible options are ReadNothingIfEventIsMissing, which skips reading entirely, or ReadEverythingIfEventIsMissing, which effectively behaves as if FromLatestEvent was not specified:

for event, err := range client.ReadEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ReadEventsOptions{
    Recursive:  false,
    FromLatestEvent: &eventsourcingdb.ReadFromLatestEvent{
      Subject:          "/books/42",
      Type:             "io.eventsourcingdb.library.book-borrowed",
      IfEventIsMissing: eventsourcingdb.ReadEverythingIfEventIsMissing,
    },
  },
) {
  // ...
}

Note that FromLatestEvent and LowerBound can not be provided at the same time.

Aborting Reading

If you need to abort reading use break or return within the for range loop. However, this only works if there is currently an iteration going on.

To abort reading independently of that, cancel the context you provided:

ctx, cancel := context.WithCancel(context.TODO())

for event, err := range client.ReadEvents(
  ctx,
  "/books/42",
  eventsourcingdb.ReadEventsOptions{
    Recursive:  false,
  },
) {
  // ...
}

// Somewhere else, cancel the context, which will cause
// reading to end.
cancel()

Running EventQL Queries

To run an EventQL query, call the RunEventQLQuery function and provide a context and the query as arguments. The function returns an iterator, which you can use e.g. inside a for range loop:

for row, err := range client.RunEventQLQuery(
  context.TODO(),
  "FROM e IN events PROJECT INTO e",
) {
  // ...
}

Note that each row returned by the iterator is of type json.RawMessage and matches the projection specified in your query.

Aborting a Query

If you need to abort a query use break or return within the for range loop. However, this only works if there is currently an iteration going on.

To abort the query independently of that, cancel the context you provided:

ctx, cancel := context.WithCancel(context.TODO())

for row, err := range client.RunEventQLQuery(
  ctx,
  "FROM e IN events PROJECT INTO e",
) {
  // ...
}

// Somewhere else, cancel the context, which will cause
// the query to end.
cancel()

Observing Events

To observe all events of a subject, call the ObserveEvents function with a context, the subject and an options object. Set the Recursive option to false. This ensures that only events of the given subject are returned, not events of nested subjects.

The function returns an iterator, which you can use e.g. inside a for range loop:

for event, err := range client.ObserveEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ObserveEventsOptions{
    Recursive: false,
  },
) {
  // ...
}

Observing From Subjects Recursively

If you want to observe not only all the events of a subject, but also the events of all nested subjects, set the Recursive option to true:

for event, err := range client.ObserveEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ObserveEventsOptions{
    Recursive: true,
  },
) {
  // ...
}

This also allows you to observe all events ever written. To do so, provide / as the subject and set Recursive to true, since all subjects are nested under the root subject.

Specifying Bounds

Sometimes you do not want to observe all events, but only a range of events. For that, you can specify the LowerBound option.

Specify the ID and whether to include or exclude it:

for event, err := range client.ObserveEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ObserveEventsOptions{
    Recursive:  false,
    LowerBound: &eventsourcingdb.Bound{
      ID:   "100",
      Type: eventsourcingdb.BoundTypeInclusive,
    },
  },
) {
  // ...
}

Starting From the Latest Event of a Given Type

To observe starting from the latest event of a given type, provide the FromLatestEvent option and specify the subject, the type, and how to proceed if no such event exists.

Possible options are WaitForEventIfEventIsMissing, which waits for an event of the given type to happen, or ObserveEverythingIfEventIsMissing, which effectively behaves as if FromLatestEvent was not specified:

for event, err := range client.ObserveEvents(
  context.TODO(),
  "/books/42",
  eventsourcingdb.ObserveEventsOptions{
    Recursive:  false,
    FromLatestEvent: &eventsourcingdb.ObserveFromLatestEvent{
      Subject:          "/books/42",
      Type:             "io.eventsourcingdb.library.book-borrowed",
      IfEventIsMissing: eventsourcingdb.ObserveEverythingIfEventIsMissing,
    },
  },
) {
  // ...
}

Note that FromLatestEvent and LowerBound can not be provided at the same time.

Aborting Observing

If you need to abort observing use break or return within the for range loop. However, this only works if there is currently an iteration going on.

To abort observing independently of that, cancel the context you provided:

ctx, cancel := context.WithCancel(context.TODO())

for event, err := range client.ObserveEvents(
  ctx,
  "/books/42",
  eventsourcingdb.ObserveEventsOptions{
    Recursive:  false,
  },
) {
  // ...
}

// Somewhere else, cancel the context, which will cause
// observing to end.
cancel()

Registering an Event Schema

To register an event schema, call the RegisterEventSchema function and hand over an event type and the desired schema:

client.RegisterEventSchema(
  "io.eventsourcingdb.library.book-acquired",
  map[string]any{
    "type": "object",
    "properties": map[string]any{
      "title":  map[string]any{ "type": "string" },
      "author": map[string]any{ "type": "string" },
      "isbn":   map[string]any{ "type": "string" },
    },
    "required": []string{
      "title",
      "author",
      "isbn",
    },
    "additionalProperties": false,
  },
)

Listing Subjects

To list all subjects, call the ReadSubjects function with a context and / as the base subject. The function returns an iterator, which you can use e.g. inside a for range loop:

for subject, err := range client.ReadSubjects(
  context.TODO(),
  "/",
) {
  // ...
}

If you only want to list subjects within a specific branch, provide the desired base subject instead:

for subject, err := range client.ReadSubjects(
  context.TODO(),
  "/books",
) {
  // ...
}

Aborting Listing

If you need to abort listing use break or return within the for range loop. However, this only works if there is currently an iteration going on.

To abort listing independently of that, cancel the context you provided:

ctx, cancel := context.WithCancel(context.TODO())

for subject, err := range client.ReadSubjects(
  ctx,
  "/",
) {
  // ...
}

// Somewhere else, cancel the context, which will cause
// reading to end.
cancel()

Listing Event Types

To list all event types, call the ReadEventTypes function. The function returns an iterator, which you can use e.g. inside a for range loop:

for eventType, err := range client.ReadEventTypes(
  context.TODO(),
) {
  // ...
}

Aborting Listing

If you need to abort listing use break or return within the for range loop. However, this only works if there is currently an iteration going on.

To abort listing independently of that, cancel the context you provided:

ctx, cancel := context.WithCancel(context.TODO())

for eventType, err := range client.ReadEventTypes(
  context.TODO(),
) {
  // ...
}

// Somewhere else, cancel the context, which will cause
// reading to end.
cancel()

Using Testcontainers

Call the NewContainer function, start the test container, defer stopping it, get a client, and run your test code:

ctx := context.TODO()

container := eventsourcingdb.NewContainer()
container.Start(ctx)
defer container.Stop(ctx)

client, err := container.GetClient(ctx)
if err != nil {
  // ...
}

// ...

To check if the test container is running, call the IsRunning function:

isRunning := container.IsRunning()

Configuring the Container Instance

By default, Container uses the latest tag of the official EventSourcingDB Docker image. To change that, call the WithImageTag function:

container := eventsourcingdb.NewContainer().
  WithImageTag("1.0.0")

Similarly, you can configure the port to use and the API token. Call the WithPort or the WithAPIToken function respectively:

container := eventsourcingdb.NewContainer().
  WithPort(4000).
  WithAPIToken("secret")

Configuring the Client Manually

In case you need to set up the client yourself, use the following functions to get details on the container:

• GetHost() returns the host name
• GetMappedPort() returns the port
• GetBaseURL() returns the full URL of the container
• GetAPIToken() returns the API token