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webdis

★ 2,961 Open GitHub ↗

A Redis HTTP interface with JSON output

// repository documentation

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# About Webdis

A very simple web server providing an HTTP interface to Redis. It embeds [hiredis](https://github.com/antirez/hiredis), [jansson](https://github.com/akheron/jansson) (with some [local changes](./src/jansson/WEBDIS-CHANGES.md)), and [http-parser](https://github.com/ry/http-parser/). It also depends on [libevent](https://monkey.org/~provos/libevent/), to be installed separately.

# Build and run from source

Building Webdis requires the libevent development package. You can install it on Ubuntu by typing `sudo apt-get install libevent-dev` or on macOS by typing `brew install libevent`.

To build Webdis with support for encrypted connections to Redis, see [Building Webdis with SSL support](#building-webdis-with-ssl-support).

```sh
$ make clean all

$ ./webdis &

$ curl http://127.0.0.1:7379/SET/hello/world
→ {"SET":[true,"OK"]}

$ curl http://127.0.0.1:7379/GET/hello
→ {"GET":"world"}

$ curl -d "GET/hello" http://127.0.0.1:7379/
→ {"GET":"world"}
```

## Configuration

Webdis is configured by a configuration file in JSON format, provided to the server as a command-line parameter:
```sh
./webdis /path/to/webdis.json
```

At start-up, Webdis will look for a file named `webdis.json` in the current directory if no file is specified on the command line.

This repository comes with a sample configuration file named `webdis.json`, to be used for local evaluation. Another file, `webdis.prod.json`, is provided as a starting point to build a production configuration file. Do **not** use either of these files in production without reviewing them.

The various features of Webdis are documented in this README.

# Try in Docker

```sh
$ docker run --name webdis-test --rm -d -p 127.0.0.1:7379:7379 nicolas/webdis
0d2ce311a4834d403cc3e7cfd571b168ba40cede6a0e155a21507bb0bf7bee81

$ curl http://127.0.0.1:7379/PING
{"PING":[true,"PONG"]}

# To stop it:
$ docker stop webdis-test
webdis-test
```

## Docker repositories and image signatures

Webdis images are published on [Docker Hub](https://hub.docker.com/r/nicolas/webdis) and [Amazon ECR](https://gallery.ecr.aws/nicolas/webdis). Starting with release `0.1.24`, images (including `latest`) are signed with [cosign](https://docs.sigstore.dev/cosign/) using the public key [`webdis.pub`](webdis.pub) at the root of this repository. For the public-key fingerprint, multi-architecture verification, and the chain of trust, see [Image signing with cosign](docs/webdis-cosign-signatures.md). Releases `0.1.12` through `0.1.23` were signed with [Docker Content Trust](docs/webdis-docker-content-trust.md), which has since been deprecated.

### Docker Hub

```sh
$ docker pull nicolas/webdis:0.1.25
$ docker pull nicolas/webdis:latest
```

Verify the signature with cosign:

```sh
$ cosign verify --key webdis.pub nicolas/webdis:0.1.25
```

When this verification is successful, you should see a message like this:

```none
Verification for public.ecr.aws/nicolas/webdis:0.1.25 --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The signatures were verified against the specified public key
```

The image is signed recursively, so verification covers the multi-arch index and each per-architecture manifest under it.

### Amazon Elastic Container Registry (ECR)

```sh
$ docker pull public.ecr.aws/nicolas/webdis:0.1.25
$ docker pull public.ecr.aws/nicolas/webdis:latest
```

ECR images share the same digests as their Docker Hub counterparts, and the cosign signatures are mirrored alongside them. The same `cosign verify` command works against ECR:

```sh
$ cosign verify --key webdis.pub public.ecr.aws/nicolas/webdis:0.1.25
```

## Multi-architecture images

Starting with [release 0.1.19](https://github.com/nicolasff/webdis/releases/tag/0.1.19), Docker images for Webdis are published as [manifest lists](https://docs.docker.com/registry/spec/manifest-v2-2/#media-types) supporting [multiple architectures](https://docs.docker.com/desktop/multi-arch/). Each release points to an x86-64 image and an ARM64v8 image:

```
$ docker manifest inspect nicolas/webdis:0.1.19 | jq -r '.manifests | .[] | .platform.architecture + " -> " + .digest'
amd64 -> sha256:2ced2d99146e1bcaf10541d17dbac573cffd02237c3b268875be1868138d3b54
arm64 -> sha256:d026c5675552947b6a755439dfd58360e44a8860436f4eddfe9b26d050801248
```

By default `docker pull` will download only the relevant image for your architecture, but you can [specify the platform](https://docs.docker.com/engine/reference/commandline/pull/) to download the image for a specific architecture, e.g.
```
$ docker pull nicolas/webdis:0.1.19 --platform linux/arm64/v8
```

# Build and run a Docker image locally

Clone the repository and open a terminal in the webdis directory, then run:
```sh
$ docker build -t webdis:custom .
[...]

$ docker run --name webdis-test --rm -d -p 127.0.0.1:7379:7379 webdis:custom
f0a2763fd456ac1f7ebff80eeafd6a5cd0fc7f06c69d0f7717fb2bdcec65926e

$ curl http://127.0.0.1:7379/PING
{"PING":[true,"PONG"]}
```

To stop it:
```
$ docker stop webdis-test
webdis-test
```

## Docker images and embedded Redis

:information_source: The Docker images [provided on Docker Hub](https://hub.docker.com/r/nicolas/webdis) under `nicolas/webdis` contain both Webdis and an embedded Redis server. They were built this way to make it easy to [try Webdis](#try-in-docker) without having to configure a Docker deployment with two containers, but this is likely not the best way to run Webdis in production.

The following documentation pages cover various such use cases:
- [Running Webdis in Docker with an external Redis instance](docs/webdis-docker-external-redis.md)
- [Running Webdis and Redis in Docker Compose](docs/webdis-redis-docker-compose.md)
- [Running Webdis and Redis in Docker Compose with SSL connections](docs/webdis-redis-docker-compose-ssl.md)

More articles are available in the [Webdis documentation](docs/README.md).

# Building Webdis with SSL support

Webdis needs libraries that provide TLS support to encrypt its connections to Redis:

* On Alpine Linux, install `openssl-dev` with `apk-add openssl-dev`.
* On Ubuntu, install `libssl-dev` with `apt-get install libssl-dev`.
* On macOS with HomeBrew, install OpenSSL with `brew install openssl@1.1`.

Then, build Webdis with SSL support enabled:

```sh
$ make SSL=1
```

# Configuring Webdis with SSL

Once Redis is configured with SSL support (see [this guide](https://nishanths.svbtle.com/setting-up-redis-with-tls) for step-by-step instructions), you can configure Webdis to connect to Redis over encrypted connections.

Add a block to `webdis.json` under a key named `"ssl"` placed at the root level, containing the following object:

```json
{
    "enabled": true,
    "ca_cert_bundle": "/path/to/ca.crt",
    "path_to_certs": "/path/to/trusted/certs",
    "client_cert": "/path/to/redis.crt",
    "client_key": "/path/to/redis.key",
    "redis_sni": "redis.mydomain.tld"
}
```
This means that `"ssl"` should be at the same level as `"redis_host"`, `"redis_port"`, etc.

**Important:** the presence of the `"ssl"` configuration block alone does not necessarily enable secure connections to Redis. The key `"enabled"` inside this block **must** also be set to `true`, otherwise Webdis will keep using unencrypted connections.

Use the following table to match the Redis configuration keys to the fields under `"ssl"` in `webdis.json`:

| Redis field        | Webdis field     | Purpose               |
| ------------------ | ---------------- | --------------------- |
| `tls-cert-file`    | `client_cert`    | Client certificate    |
| `tls-key-file`     | `client_key`     | Client key            |
| `tls-ca-cert-file` | `ca_cert_bundle` | CA certificate bundle |

Two other keys have no equivalent in `redis.conf`:

- `path_to_certs` is an optional directory path where trusted CA certificate files are stored in an OpenSSL-compatible format.
- `redis_sni` is an optional Redis server name, used as a server name indication (SNI) TLS extension.

See also the [Hiredis docs](https://github.com/redis/hiredis/blob/v1.0.2/README.md#hiredis-openssl-wrappers) and [Hiredis source code](https://github.com/redis/hiredis/blob/v1.0.2/hiredis_ssl.h#L77-L96) for more information.

### Running Redis and Webdis with SSL in Docker Compose

For a full tutorial showing how to configure and run Redis and Webdis under Docker Compose with SSL connections between the two services, head to the `docs` folder and open [Running Webdis & Redis in Docker Compose with SSL connections](docs/webdis-redis-docker-compose-ssl.md).

## SSL troubleshooting

Follow this table to diagnose issues with SSL connections to Redis.

| Error message or issue | Cause | Solution |
| ---------------------- | ----- | -------- |
| Unexpected key or incorrect value in `webdis.json`: 'ssl' | Webdis is not compiled with SSL support | Build webdis with `make SSL=1` |
| Unexpected key or incorrect value under 'ssl' | Invalid configuration | One or more keys in the `ssl` object in was not recognized, make sure they are all valid |
| Failed to load client certificate | Invalid client certificate | Verify the file that `client_cert` points to |
| Failed to load private key | Invalid client key | Verify the file that `client_key` points to |
| Failed to load CA Certificate or CA Path | Invalid CA certificate bundle | Verify the file that `ca_cert_bundle` points to |
| All requests fail with HTTP 503, logs show "Error disconnecting: Connection reset by peer" | SSL disabled in config but Webdis connected to an SSL port | Make sure `enabled` is set to `true` and that Webdis connects to the SSL port for Redis |
| Logs show "Server closed the connection" at start-up | SSL connection failed | The client key and/or client certificate was missing. Make sure the configuration is valid. |
| No error but all requests hang | Webdis connected to the non-SSL port | Make sure Webdis is connecting to the port set under `tls-port` in `redis.conf` |

# Features
* `GET` and `POST` are supported, as well as `PUT` for file uploads (see example of `PUT` usage [here](#file-upload)).
* JSON output by default, optional JSONP parameter (`?jsonp=myFunction` or `?callback=myFunction`).
* Raw Redis 2.0 protocol output with `.raw` suffix.
* MessagePack output with `.msg` suffix.
* HTTP 1.1 pipelining (70,000 http requests per second on a desktop Linux machine.)
* Multi-threaded server, configurable number of worker threads.
* [WebSocket support](#websockets) (Currently using the specification from [RFC 6455](https://datatracker.ietf.org/doc/html/rfc6455)).
* Connects to Redis using a TCP or UNIX socket.
* Support for [secure connections to Redis](#configuring-webdis-with-ssl) (requires [Redis 6 or newer](https://redis.io/topics/encryption)).
* Support for "Keep-Alive" connections to Redis: add `"hiredis": { "keep_alive_sec": 15 }` to `webdis.json` to enable it with the default value. See the [Hiredis documentation](https://github.com/redis/hiredis/tree/e07ae7d3b6248be8be842eca3e1e97595a17aa1a#other-configuration-using-socket-options) for details, the value configured in `webdis.json` is the `interval` passed to `redisEnableKeepAliveWithInterval`. Important: note how it is used to set the value for `TCP_KEEPALIVE` (the same value) _and_ to compute the value for `TCP_KEEPINTVL` (integer, set to 1/3 × `interval`).
* Restricted commands by IP range (CIDR subnet + mask) or HTTP Basic Auth, returning 403 errors.
* Support for Redis authentication in the config file: set `redis_auth` to a single string to use a password value, or to an array of two strings to use username+password auth ([new in Redis 6.0](https://redis.io/commands/auth)).
* Environment variables can be used as values in the config file, starting with `$` and in all caps (e.g. `$REDIS_HOST`).
* Pub/Sub using `Transfer-Encoding: chunked`, works with JSONP as well. Webdis can be used as a Comet server.
* Drop privileges on startup.
* Custom Content-Type using a pre-defined file extension, or with `?type=some/thing`.
* URL-encoded parameters for binary data or slashes and question marks. For instance, `%2f` is decoded as `/` but not used as a command separator.
* Logs, with a configurable verbosity.
* Configurable `fsync` frequency for the log file:
    * Set `"log_fsync": "auto"` (default) to let the file system handle file persistence on its own.
    * Set `"log_fsync": N` where `N` is a number to call `fsync` every `N` milliseconds.
    * Set `"log_fsync": "all"` (very slow) to persist the log file to its storage device on each log message.
* Reload the config file on SIGHUP, for compatibility with logrotate, systemd, etc.
* Cross-origin requests, usable with XMLHttpRequest2 (Cross-Origin Resource Sharing - CORS).
* [File upload](#file-upload) with `PUT`.
* With the JSON output, the return value of INFO is parsed and transformed into an object.
* Optionally run as a daemon process: set `"daemonize": true` and `"pidfile": "/var/run/webdis.pid"` in `webdis.json`.
* Default root object: Add `"default_root": "/GET/index.html"` in `webdis.json` to substitute the request to `/` with a Redis request.
* HTTP request limit with `http_max_request_size` (in bytes, set to 128 MB by default).
* Database selection in the URL, using e.g. `/7/GET/key` to run the command on DB 7.

# Ideas, TODO…
* Add better support for PUT, DELETE, HEAD, OPTIONS? How? For which commands?
    * This could be done using a “strict mode” with a table of commands and the verbs that can/must be used with each command. Strict mode would be optional, configurable. How would webdis know of new commands remains to be determined.
* Enrich config file:
    * Provide timeout (maybe for some commands only?). What should the response be? 504 Gateway Timeout? 503 Service Unavailable?
* Multi-server support, using consistent hashing.
* Please send your ideas as GitHub issues.

# Past considerations
* `MULTI`/`EXEC`: the same benefits are available with more flexibility using the `EVAL`/`EVALSHA` commands. See [this comment](https://github.com/nicolasff/webdis/issues/263#issuecomment-3949305136) for more details.
* Support for SSL/TLS in Webdis: other servers such as nginx or haproxy are better suited to terminate SSL/TLS. It is easy to find documentation and examples online. The goal of Webdis is not to attempt to replace nginx while providing fewer features and worse performance.

# HTTP error codes
* Unknown HTTP verb: 405 Method Not Allowed.
* Redis is unreachable: 503 Service Unavailable.
* Matching ETag sent using `If-None-Match`: 304 Not Modified.
* Could also be used:
    * Timeout on the redis side: 503 Service Unavailable.
    * Missing key: 404 Not Found.
    * Unauthorized command (disabled in config file): 403 Forbidden.

# Command format
The URI `/COMMAND/arg0/arg1/.../argN.ext` executes the command on Redis and returns the response to the client. GET, POST, and PUT are supported:

* `GET /COMMAND/arg0/.../argN.ext`
* `POST /` with `COMMAND/arg0/.../argN` in the HTTP body.
* `PUT /COMMAND/arg0.../argN-1` with `argN` in the HTTP body (see section on [file uploads](#file-upload).)

`.ext` is an optional extension; it is not read as part of the last argument but only represents the output format. Several formats are available (see below).

Special characters: `/` and `.` have special meanings, `/` separates arguments and `.` changes the Content-Type. They can be replaced by `%2f` and `%2e`, respectively.

# Redis authentication

Webdis can connect to a Redis server that requires credentials.
For Redis versions before 6.0, provide the password as a single string in `webdis.json` using the key `"redis_auth"`. For example:
```json
    "redis_auth": "enter-password-here"
```
Redis 6.0 introduces a more granular [access control system](https://redis.io/topics/acl) and switches from a single password to a pair of username and password. To use these two values with Webdis, set `"redis_auth"` to an array containing the two strings, e.g.
```json
    "redis_auth": ["my-username", "my-password"]
```
This new authentication system is only supported in Webdis 0.1.13 and above.

# ACL
Access control is configured in `webdis.json`. Each configuration tries to match a client profile according to two criteria:

* [CIDR](https://en.wikipedia.org/wiki/CIDR) subnet + mask
* [HTTP Basic Auth](https://en.wikipedia.org/wiki/Basic_access_authentication) in the format of "user:password".

Each ACL contains two lists of commands, `enabled` and `disabled`. All commands being enabled by default, it is up to the administrator to disable or re-enable them on a per-profile basis.

Examples:
```json
{
    "disabled": ["DEBUG", "FLUSHDB", "FLUSHALL"],
},

{
    "http_basic_auth": "user:password",
    "disabled":        ["DEBUG", "FLUSHDB", "FLUSHALL"],
    "enabled":         ["SET"]
},

{
    "ip":      "192.168.10.0/24",
    "enabled": ["SET"]
},

{
    "http_basic_auth": "user:password",
    "ip":              "192.168.10.0/24",
    "enabled":         ["SET", "DEL"]
}
```
ACLs are interpreted in order, later authorizations superseding earlier ones if a client matches several. The special value "*" matches all commands.

## ACLs and Websocket clients

These rules apply to WebSocket connections as well, although without support for HTTP Basic Auth filtering. IP filtering is supported.

For JSON-based WebSocket clients, a rejected command will return this object (sent as a string in a binary frame):
```json
{"message": "Forbidden", "error": true, "http_status": 403}
```
The `http_status` code is an indicator of how Webdis would have responded if the client had used HTTP instead of a WebSocket connection, since WebSocket messages do not inherently have a status code.

For raw Redis protocol WebSocket clients, a rejected command will produce this error (sent as a string in a binary frame):
```
-ERR Forbidden\r\n
```

# Environment variables

Environment variables can be used in `webdis.json` to read values from the environment instead of using constant values.
For this, the value must be a string starting with a dollar symbol and written in all caps. For example, to make the redis host and port configurable via environment variables, use the following:

```json
{
    "redis_host": "$REDIS_HOST",
    "redis_port": "$REDIS_PORT",
}
```

# JSON output
JSON is the default output format. Each command returns a JSON object with the command as a key and the result as a value.

**Examples:**
```sh
// string
$ curl http://127.0.0.1:7379/GET/y
{"GET":"41"}

// number
$ curl http://127.0.0.1:7379/INCR/y
{"INCR":42}

// list
$ curl http://127.0.0.1:7379/LRANGE/x/0/1
{"LRANGE":["abc","def"]}

// status
$ curl http://127.0.0.1:7379/TYPE/y
{"TYPE":[true,"string"]}

// error, which is basically a status
$ curl http://127.0.0.1:7379/MAKE-ME-COFFEE
{"MAKE-ME-COFFEE":[false,"ERR unknown command 'MAKE-ME-COFFEE'"]}

// JSONP callback:
$ curl  "http://127.0.0.1:7379/TYPE/y?jsonp=myCustomFunction"
myCustomFunction({"TYPE":[true,"string"]})
```

# RAW output
This is the raw output of Redis; enable it with the `.raw` suffix.
```sh
// string
$ curl http://127.0.0.1:7379/GET/z.raw
$5
hello

// number
$ curl http://127.0.0.1:7379/INCR/a.raw
:2

// list
$ curl http://127.0.0.1:7379/LRANGE/x/0/-1.raw
*2
$3
abc
$3
def

// status
$ curl http://127.0.0.1:7379/TYPE/y.raw
+zset

// error, which is basically a status
$ curl http://127.0.0.1:7379/MAKE-ME-COFFEE.raw
-ERR unknown command 'MAKE-ME-COFFEE'
```

# Custom content-type
Several content-types are available:

* `.json` for `application/json` (this is the default Content-Type).
* `.msg` for `application/x-msgpack`. See [https://msgpack.org/](https://msgpack.org/) for the specs.
* `.txt` for `text/plain`
* `.html` for `text/html`
* `.xhtml` for `application/xhtml+xml`
* `.xml` for `text/xml`
* `.png` for `image/png`
* `.jpg` or `.jpeg` for `image/jpeg`
* Any other with the `?type=anything/youwant` query string.
* Add a custom separator for list responses with `?sep=,` query string.

```
$ curl -v "http://127.0.0.1:7379/GET/hello.html"
[...]
< HTTP/1.1 200 OK
< Content-Type: text/html
< Date: Mon, 03 Jan 2011 20:43:36 GMT
< Content-Length: 137
<
<!DOCTYPE html>
<html>
[...]
</html>

$ curl -v "http://127.0.0.1:7379/GET/hello.txt"
[...]
< HTTP/1.1 200 OK
< Content-Type: text/plain
< Date: Mon, 03 Jan 2011 20:43:36 GMT
< Content-Length: 137
[...]

$ curl -v "http://127.0.0.1:7379/GET/big-file?type=application/pdf"
[...]
< HTTP/1.1 200 OK
< Content-Type: application/pdf
< Date: Mon, 03 Jan 2011 20:45:12 GMT
[...]
```

# File upload
Webdis supports file upload using HTTP PUT. The command URI is slightly different, as the last argument is taken from the HTTP body.
For example: instead of `/SET/key/value`, the URI becomes `/SET/key` and the value is the entirety of the body. This works for other commands such as LPUSH, etc.

**Uploading a binary file to webdis**:
```
$ file redis-logo.png
redis-logo.png: PNG image, 513 x 197, 8-bit/color RGBA, non-interlaced

$ wc -c redis-logo.png
16744 redis-logo.png

$ curl -v --upload-file redis-logo.png http://127.0.0.1:7379/SET/logo
[...]
> PUT /SET/logo HTTP/1.1
> User-Agent: curl/7.19.7 (x86_64-pc-linux-gnu) libcurl/7.19.7 OpenSSL/0.9.8k zlib/1.2.3.3 libidn/1.15
> Host: 127.0.0.1:7379
> Accept: */*
> Content-Length: 16744
> Expect: 100-continue
>
< HTTP/1.1 100 Continue
< HTTP/1.1 200 OK
< Content-Type: application/json
< ETag: "0db1124cf79ffeb80aff6d199d5822f8"
< Date: Sun, 09 Jan 2011 16:48:19 GMT
< Content-Length: 19
<
{"SET":[true,"OK"]}

$ curl -vs http://127.0.0.1:7379/GET/logo.png -o out.png
> GET /GET/logo.png HTTP/1.1
> User-Agent: curl/7.19.7 (x86_64-pc-linux-gnu) libcurl/7.19.7 OpenSSL/0.9.8k zlib/1.2.3.3 libidn/1.15
> Host: 127.0.0.1:7379
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: image/png
< ETag: "1991df597267d70bf9066a7d11969da0"
< Date: Sun, 09 Jan 2011 16:50:51 GMT
< Content-Length: 16744

$ md5sum redis-logo.png out.png
1991df597267d70bf9066a7d11969da0  redis-logo.png
1991df597267d70bf9066a7d11969da0  out.png
```

The file was uploaded and re-downloaded properly: it has the same hash and the content-type was set properly thanks to the `.png` extension.

# WebSockets
Webdis supports WebSocket clients implementing [RFC 6455](https://datatracker.ietf.org/doc/html/rfc6455).

**Important:** WebSocket support is currently _disabled by default_. To enable WebSocket support, set the key named `"websockets"` to value `true` in `webdis.json`, e.g.

```json
{
    "daemonize": false,
    "websockets": true,
}
```
(start and end of file omitted).

WebSockets are supported with the following formats, selected by the connection URL:

* JSON (on `/` or `/.json`)
* Raw Redis wire protocol (on `/.raw`)

**Example**:
```javascript
function testJSON() {
    var jsonSocket = new WebSocket("ws://127.0.0.1:7379/.json");
    jsonSocket.onmessage = function(messageEvent) {
        console.log("JSON received:", messageEvent.data);
    };
    jsonSocket.onopen = function() {
        console.log("JSON socket connected!");
        jsonSocket.send(JSON.stringify(["SET", "hello", "world"]));
        jsonSocket.send(JSON.stringify(["GET", "hello"]));
    };
}
testJSON();
```

This produces the following output:
```
JSON socket connected!
JSON received: {"SET":[true,"OK"]}
JSON received: {"GET":"world"}
```

## WebSockets HTML demo

The Webdis repository contains a demo web page with JavaScript code that can be used to test WebSocket support.

In a terminal, check out Webdis, build it, and configure it with WebSocket support:

```shell
$ cd ~/src/webdis
$ make
$ vim webdis.json      # (edit the file to add "websockets": true)
$ grep websockets webdis.json
    "websockets": true,
$ ./webdis
```

Then go to the `tests/` directory and open `websocket.html` with a web browser.

# Pub/Sub with chunked transfer encoding
Webdis exposes Redis PUB/SUB channels to HTTP clients, forwarding messages in the channel as they are published by Redis. This is done using chunked transfer encoding.

**Example using XMLHttpRequest**:
```javascript
var previous_response_length = 0
xhr = new XMLHttpRequest()
xhr.open("GET", "http://127.0.0.1:7379/SUBSCRIBE/hello", true);
xhr.onreadystatechange = checkData;
xhr.send(null);

function checkData() {
    if(xhr.readyState == 3)  {
        response = xhr.responseText;
        chunk = response.slice(previous_response_length);
        previous_response_length = response.length;
        console.log(chunk);
    }
};
```

Publish messages to redis to see output similar to the following:
```json
{"SUBSCRIBE":["subscribe","hello",1]}
{"SUBSCRIBE":["message","hello","some message"]}
{"SUBSCRIBE":["message","hello","some other message"]}
```

webdis

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