# How does consul achieve HA

**URL:** <https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908>\
**Category:** Consul\
**Created:** [April 30, 2021, 6:22pm UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908 "2021-04-30T18:22:51Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![murugaratham](https://sea2.discourse-cdn.com/hashicorp/user_avatar/discuss.hashicorp.com/murugaratham/32/19090_2.png) [@murugaratham](https://discuss.hashicorp.com/u/murugaratham)\
**Post date:** [April 30, 2021, 6:22pm UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908/1 "2021-04-30T18:22:51Z")

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I’m confused with achieving high availability for cluster on docker.

On GitHub, there’s a demo cluster compose file:

version: ‘3’

services:

consul-agent-1: &consul-agent  
image: consul:latest  
networks:  
- consul-demo  
command: “agent -retry-join consul-server-bootstrap -client 0.0.0.0”

consul-agent-2:  
\<\<: \*consul-agent

consul-agent-3:  
\<\<: \*consul-agent

consul-server-1: &consul-server  
\<\<: \*consul-agent  
command: “agent -server -retry-join consul-server-bootstrap -client 0.0.0.0”

consul-server-2:  
\<\<: \*consul-server

consul-server-bootstrap:  
\<\<: \*consul-agent  
ports:  
- “8400:8400”  
- “8500:8500”  
- “8600:8600”  
- “8600:8600/udp”  
command: “agent -server -bootstrap-expect 3 -ui -client 0.0.0.0”

networks:  
consul-demo:

So what happens when consul-server-bootstrap container goes down for what ever reason?

I’m using .net core which registers services with http and I’m going to use consul-server-bootstrap:8500

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**Author:** ![Ranjandas](https://avatars.discourse-cdn.com/v4/letter/r/90ced4/32.png) [@Ranjandas](https://discuss.hashicorp.com/u/Ranjandas)\
**Post date:** [May 4, 2021, 1:04am UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908/2 "2021-05-04T01:04:17Z")

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Hi @murugaratham,

Welcome to the forums!

In this example, there are 3 Consul Server Agents and 3 Consul Client Agents. When the cluster gets bootstrapped, one of the Server Agents will become the Leader of the cluster. In a cluster with 3 Server nodes, you can at most lose 1 Server and still have the cluster healthy. So to answer your question your cluster will be fine even if the `consul-server-bootstrap` container goes down.

To get a full understanding of how this works, you will have to start reading from the architecture documentation, followed by the most important pieces, the [Consensus Protocol](https://www.consul.io/docs/architecture/consensus) and [Gossip Protocol](https://www.consul.io/docs/architecture/gossip).

- [Consul Architecture | Consul by HashiCorp](https://www.consul.io/docs/architecture)

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<div class="post-metadata">

**Author:** ![murugaratham](https://sea2.discourse-cdn.com/hashicorp/user_avatar/discuss.hashicorp.com/murugaratham/32/19090_2.png) [@murugaratham](https://discuss.hashicorp.com/u/murugaratham)\
**Post date:** [May 4, 2021, 1:30am UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908/3 "2021-05-04T01:30:23Z")

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Hi @Ranjandas,

However in the sample yml file, the ports being mapped is limited on consul-server-bootstrap container only, if it does down, how would the other containers be able to connect to the ports though

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<div class="post-metadata">

**Author:** ![Ranjandas](https://avatars.discourse-cdn.com/v4/letter/r/90ced4/32.png) [@Ranjandas](https://discuss.hashicorp.com/u/Ranjandas)\
**Post date:** [May 4, 2021, 1:44am UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908/4 "2021-05-04T01:44:32Z")

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In the example compose file, the expectation is that all Consul clients are attached to the same network `consul-demo`. As a result, all Consul agents will be able to talk to each other without any port mapping.

The port mapping you are seeing for `consul-server-bootstrap` container is just for you to interact with the cluster over `8500` (Consul HTTP API) and `8600` (Consul DNS Port) (btw 8400 is deprecated and not required) from your docker host.

After all, this compose file is only intended for a development environment. And while developing, if the `consul-server-bootstrap` container fails for some reason, you won’t be able to communicate with the cluster, unless you exec into one of the rest of the consul nodes and run Consul CLI.

Hope this helps.

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<div class="post-metadata">

**Author:** ![murugaratham](https://sea2.discourse-cdn.com/hashicorp/user_avatar/discuss.hashicorp.com/murugaratham/32/19090_2.png) [@murugaratham](https://discuss.hashicorp.com/u/murugaratham)\
**Post date:** [May 4, 2021, 1:57am UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908/5 "2021-05-04T01:57:41Z")

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I see, as i was planning to create a HA consul, and have application containers connect to consul for service discovery and was confused by the port mapping on 1 (point of failure) bootstrap agent, well thanks for the clarifications though!

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<div class="post-metadata">

**Author:** ![Ranjandas](https://avatars.discourse-cdn.com/v4/letter/r/90ced4/32.png) [@Ranjandas](https://discuss.hashicorp.com/u/Ranjandas)\
**Post date:** [May 4, 2021, 2:22am UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908/6 "2021-05-04T02:22:20Z")

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One thing to note is `consul-server-bootstrap` container is not a point of failure once the cluster is formed. There is no guarantee that it would become the `leader`. This is perfectly ok as in Consul you don’t need to explicitly talk to the `leader` to talk to the Cluster (RPC requests will get forward to the leader as required).

The reason why only one container has the port mapping is that once you add for other two containers, you will have to randomize the host ports and this, in turn, will require you to expose `CONSUL_HTTP_ADDR` every time on the host to talk to the right `<host>:<port>` combination when the container fails.

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<div class="post-metadata">

**Author:** ![murugaratham](https://sea2.discourse-cdn.com/hashicorp/user_avatar/discuss.hashicorp.com/murugaratham/32/19090_2.png) [@murugaratham](https://discuss.hashicorp.com/u/murugaratham)\
**Post date:** [May 4, 2021, 2:36am UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908/7 "2021-05-04T02:36:21Z")

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so from docker perspective, how does application containers connect to consul? Right now i’m providing bootstrap agent’s address to containers

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<div class="post-metadata">

**Author:** ![Ranjandas](https://avatars.discourse-cdn.com/v4/letter/r/90ced4/32.png) [@Ranjandas](https://discuss.hashicorp.com/u/Ranjandas)\
**Post date:** [May 4, 2021, 2:55am UTC](https://discuss.hashicorp.com/t/how-does-consul-achieve-ha/23908/8 "2021-05-04T02:55:28Z")

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In production scenario, you would list all the server nodes in the `-retry-join` list to avoid the case of single point of failure (in this case only `consul-server-bootstrap` is given). This could be IP Addresses, Hostnames or even dynamic lookups (ref: [Cloud Auto-join | Consul by HashiCorp](https://www.consul.io/docs/install/cloud-auto-join)).

Again this docker-compose file is aimed at dev setup, I wouldn’t consider this as production ready.