Semaphore usage

The eridu-tech/semaphore component provides a way for managing semaphores independent of underlying platform or storage.

Initial configuration

To begin using the SemaphoreFactory class, you'll need to create and configure an instance:

import { TimeSpan } from "eridu-tech/time-span";
import { MemorySemaphoreAdapter } from "eridu-tech/semaphore/memory-semaphore-adapter";
import { SemaphoreFactory } from "eridu-tech/semaphore";

const semaphoreFactory = new SemaphoreFactory({
    // You can provide default TTL value
    // If you set it to null it means semaphores will not expire and most be released manually by default.
    defaultTtl: TimeSpan.fromSeconds(2),

    // You can choose the adapter to use
    adapter: new MemorySemaphoreAdapter(),
});

info

Here is a complete list of settings for the SemaphoreFactory class.

Semaphore basics

Creating a semaphore

const semaphore = semaphoreFactory.create("shared-resource", {
    // You need to define a limit
    limit: 2,
});

Acquiring and releasing the semaphore

// 1 slot will be acquired
if (await semaphore.acquire()) {
    console.log("Acquired");
    try {
        // The concurrent section
    } finally {
        await semaphore.release();
    }
} else {
    console.log("Unable to acquire");
}

// 2 slots will be acquired
if (await semaphore.acquire()) {
    console.log("Acquired");
    try {
        // The concurrent section
    } finally {
        await semaphore.release();
    }
} else {
    console.log("Unable to acquire");
}

// Will log false because the limit is reached
console.log(await semaphore.acquire());

Alternatively you could write it as follows:

// 1 slot will be acquired
try {
    console.log("Acquired");
    // This method will throw if the semaphore limit is reached.
    await semaphore.acquireOrFail();
    // The critical section
} catch {
    console.log("Unable to acquire");
} finally {
    await semaphore.release();
}

// 2 slots will be acquired
try {
    console.log("Acquired");
    // This method will throw if the semaphore limit is reached.
    await semaphore.acquireOrFail();
    // The critical section
} catch {
    console.log("Unable to acquire");
} finally {
    await semaphore.release();
}

// Will throw because the limit is reached
await semaphore.acquireOrFail();

danger

You need always to wrap the concurrent section with try-finally so the semaphore get released when error occurs.

Semaphore with custom TTL

You can provide a custom TTL for the semaphore.

const semaphore = semaphoreFactory.create("shared-resource", {
    // Default TTL is 5min if not overrided
    // If you set it to null it means semaphore will not expire and most be released manually.
    ttl: TimeSpan.fromSeconds(30),
    limit: 2,
});

Checking semaphore state

You can get the semaphore state by using the getState method, it returns ISemaphoreState.

import { SEMAPHORE_STATE } from "eridu-tech/semaphore/contracts";

const semaphore = semaphoreFactory.create("shared-resource", {
    limit: 2,
});
const state = await semaphore.getState();

if (state.type === SEMAPHORE_STATE.EXPIRED) {
    console.log("The semaphore doesnt exists");
}

if (state.type === SEMAPHORE_STATE.LIMIT_REACHED) {
    console.log("The limit have been reached and all slots are unavailable");
}

if (state.type === SEMAPHORE_STATE.ACQUIRED) {
    console.log("The semaphore is acquired");
}

if (state.type === SEMAPHORE_STATE.UNACQUIRED) {
    console.log("There are avilable slots but the semaphore is not acquired");
}

Patterns

Refreshing semaphores

The semaphore can be refreshed by the current owner before it expires. This is particularly useful for long-running tasks, instead of setting an excessively long TTL initially, you can start with a shorter one and use the refresh method to set the TTL of the semaphore:

import { delay } from "eridu-tech/utilities";

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
    ttl: TimeSpan.fromMinutes(1),
});

async function doWork(): Promise<boolean> {
    // ... critical section
}

const hasAcquired = await semaphore.acquire();
if (hasAcquired) {
    try {
        while (true) {
            await semaphore.refresh(TimeSpan.fromMinutes(1));
            const hasFinished = await doWork();
            if (hasFinished) {
                break;
            }
            await delay(TimeSpan.fromSeconds(1));
        }
    } finally {
        await semaphore.release();
    }
}

warning

Note: A semaphore must have an expiration (a ttl value) to be refreshed. You cannot refresh a semaphore that was created without an expiration (with ttl: null)

// Create a semaphore with no expiration (non-refreshable)
const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
    ttl: null,
});

// A refresh attempt on this semaphore will fail
const hasRefreshed = await semaphore.refresh();

// This will log 'false' because the semaphore cannot be refreshed
console.log(hasRefreshed);

Additional methods

The releaseOrFail method is the same release method but it throws an error when not enable to release the semaphore:

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});

await semaphore.releaseOrFail();

You can force release all the semaphore slots:

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});

await semaphore.forceReleaseAll();

The refreshOrFail method is the same refresh method but it throws an error when not enable to refresh the semaphore:

const semaphore = semaphoreFactory.create("resource");

await semaphore.refreshOrFail();

The runOrFail method automatically manages semaphore acquisition and release around function execution. It calls acquireOrFail before invoking the function and calls release in a finally block, ensuring the semaphore is always freed, even if an error occurs during execution.

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});

await semaphore.runOrFail(async () => {
    // ... critical section
});

info

Note the method throws an error when the semaphore cannot be acquired.

info

You can provide synchronous or asynchronous Invocable<[], TValue | Promise<TValue>> as values for the runOrFail method.

Semaphore instance variables

The Semaphore class exposes instance variables such as:

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});

// Will return the key of the semaphore which is "resource"
console.log(semaphore.key.toString());

// Will return the id of the semaphore
console.log(semaphore.id);

// Will return the ttl of the semaphore
console.log(semaphore.ttl);

Semaphore slot id

By default the slot id is autogenerated but it can also manually defined.

const semaphore = semaphoreFactory.create("semaphore", {
    slotId: "my-slot-id",
});

const hasAcquire = await semaphore.acquire();
if (hasAcquired) {
    console.log("Shared resource");
    await semaphore.release();
}

info

Manually defining slot id is primarily useful for debugging or implementing manual resource controll by the end user.

warning

In most cases, setting a slot id is unnecessary.

Retrying acquiring semaphore by attempts

To retry acquiring semaphore you can use the retry middleware.

Retrying acquiring semaphore with acquireOrFail method:

import { retry } from "eridu-tech/resilience";
import { FailedAcquireSemaphoreError } from "eridu-tech/semaphore/contracts";
import { use } from "eridu-tech/middleware";

const semaphore = semaphoreFactory.create("semaphore", {
    limit: 2,
});

try {
    await use(async () => {
        await semaphore.acquireOrFail();
    }, [
        retry({
            maxAttempts: 4,
            errorPolicy: FailedAcquireSemaphoreError,
        }),
    ])();
    // The critical section
} finally {
    await semaphore.release();
}

Retrying acquiring semaphore with acquire method:

import { retry } from "eridu-tech/resilience";
import { use } from "eridu-tech/middleware";

const semaphore = semaphoreFactory.create("semaphore", {
    limit: 2,
});

const hasAquired = await use(async () => {
    return await semaphore.acquire();
}, [
    retry({
        maxAttempts: 4,
        errorPolicy: {
            treatFalseAsError: true,
        },
    }),
])();

if (hasAquired) {
    try {
        // The critical section
    } finally {
        await semaphore.release();
    }
}

Retrying acquiring semaphore with runOrFail method:

import { retry } from "eridu-tech/resilience";
import { FailedAcquireSemaphoreError } from "eridu-tech/semaphore/contracts";
import { use } from "eridu-tech/middleware";

const semaphore = semaphoreFactory.create("semaphore", {
    limit: 2,
});

await use(async () => {
    await semaphore.runOrFail(async () => {
        // The critical section
    });
}, [
    retry({
        maxAttempts: 4,
        errorPolicy: FailedAcquireSemaphoreError,
    }),
])();

Retrying acquiring semaphore by interval

To retry acquiring semaphore at regular intervals you can use the retryInterval middleware:

Retrying acquiring semaphore with acquireOrFail method:

import { retryInterval } from "eridu-tech/resilience";
import { FailedAcquireSemaphoreError } from "eridu-tech/semaphore/contracts";
import { use } from "eridu-tech/middleware";
import { TimeSpan } from "eridu-tech/time-span";

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});

try {
    await use(async () => {
        await semaphore.acquireOrFail();
    }, [
        retryInterval({
            // Time to wait 1 minute
            time: TimeSpan.fromMinutes(1),
            // Interval to try acquire the semaphore
            interval: TimeSpan.fromSeconds(1),
            errorPolicy: FailedAcquireSemaphoreError,
        }),
    ])();
    // ... critical section
} finally {
    await semaphore.release();
}

Retrying acquiring semaphore with acquire method:

import { retryInterval } from "eridu-tech/resilience";
import { use } from "eridu-tech/middleware";
import { TimeSpan } from "eridu-tech/time-span";

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});

const hasAcquired = await use(async () => {
    return await semaphore.acquire();
}, [
    retryInterval({
        time: TimeSpan.fromMinutes(1),
        interval: TimeSpan.fromSeconds(1),
        errorPolicy: {
            treatFalseAsError: true,
        },
    }),
])();

if (hasAcquired) {
    try {
        // ... critical section
    } finally {
        await semaphore.release();
    }
}

Retrying acquiring semaphore with runOrFail method:

import { retryInterval } from "eridu-tech/resilience";
import { FailedAcquireSemaphoreError } from "eridu-tech/semaphore/contracts";
import { use } from "eridu-tech/middleware";
import { TimeSpan } from "eridu-tech/time-span";

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});

await use(async () => {
    await semaphore.runOrFail(async () => {
        // ... critical section
    });
}, [
    retryInterval({
        time: TimeSpan.fromMinutes(1),
        interval: TimeSpan.fromSeconds(1),
        errorPolicy: FailedAcquireSemaphoreError,
    }),
])();

warning

Note using retryInterval middleware with semaphore acquiring in a HTTP request handler is discouraged because it blocks the HTTP request handler causing the handler wait until the semaphore becomes available or the timeout is reached. This will delay HTTP request handler to generate response and will make frontend app slow because of HTTP request handler.

Serialization and deserialization of semaphore

Semaphores can be serialized, allowing them to be transmitted over the network to another server and later deserialized for reuse. This means you can, for example, acquire the semaphore on the main server, transfer it to a queue worker server, and release it there. In order to serialize or deserialize a semaphore you need pass an object that implements ISerderRegister contract like the Serde class to SemaphoreFactory.

Manually serializing and deserializing the semaphore:

import { RedisSemaphoreAdapter } from "eridu-tech/semaphore/redis-semaphore-adapter";
import { SemaphoreFactory } from "eridu-tech/semaphore";
import { Serde } from "eridu-tech/serde";
import { SuperJsonSerdeAdapter } from "eridu-tech/serde/super-json-serde-adapter";

const serde = new Serde(new SuperJsonSerdeAdapter());

const redisClient = new Redis("YOUR_REDIS_CONNECTION");

const semaphoreFactory = new SemaphoreFactory({
    // You can laso pass in an array of Serde class instances
    serde,
    adapter: new RedisSemaphoreAdapter(redisClient),
});

const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});
const serializedSemaphore = serde.serialize(semaphore);
const deserializedSemaphore = serde.deserialize(semaphore);

danger

When serializing or deserializing a semaphore, you must use the same Serde instances that were provided to the SemaphoreFactory. This is required because the SemaphoreFactory injects custom serialization logic for ISemaphore instance into Serde instances.

info

Note you only need manuall serialization and deserialization when integrating with external libraries.

As long you pass the same Serde instances with all other components you dont need to serialize and deserialize the semaphore manually.

import { RedisSemaphoreAdapter } from "eridu-tech/semaphore/redis-semaphore-adapter";
import type { ISemaphore } from "eridu-tech/semaphore/contracts";
import { SemaphoreFactory } from "eridu-tech/semaphore";
import { RedisPubSubEventBusAdapter } from "eridu-tech/event-bus/redis-pub-sub-event-bus-adapter";
import { EventBus } from "eridu-tech/event-bus";
import { Serde } from "eridu-tech/serde";
import { SuperJsonSerdeAdapter } from "eridu-tech/serde/super-json-serde-adapter";

const serde = new Serde(new SuperJsonSerdeAdapter());
const redis = new Redis("YOUR_REDIS_CONNECTION");

type EventMap = {
    "sending-semaphore-over-network": {
        semaphore: ISemaphore;
    };
};
const eventBus = new EventBus<EventMap>({
    adapter: new RedisPubSubEventBusAdapter({
        client: redis,
        serde,
    }),
});

const semaphoreFactory = new SemaphoreFactory({
    serde,
    adapter: new RedisSemaphoreAdapter(redis),
    eventBus,
});
const semaphore = semaphoreFactory.create("resource", {
    limit: 2,
});

// We are sending the semaphore over the network to other servers.
await eventBus.dispatch("sending-semaphore-over-network", {
    semaphore,
});

// The other servers will recieve the serialized semaphore and automattically deserialize it.
await eventBus.addListener(
    "sending-semaphore-over-network",
    ({ semaphore }) => {
        // The semaphore is deserialized and can be used
        console.log("SEMAPHORE:", semaphore);
    },
);

Separating semaphore creation from manipulation

The library includes 2 additional contracts:

• ISemaphore - Allows only for manipulating of the semaphore.

• ISemaphoreFactory - Allows only for creation of semaphores.

Further information

For further information refer to eridu-tech/semaphore API docs.