Adds zombienet overview

Author: amatsonkali

docs/zombienet/before-completed-pod-err.png

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+# Zombienet overview
+
+- [Zombienet overview](#zombienet-overview)
+  - [Requirements](#requirements)
+  - [Initial setup](#initial-setup)
+  - [Creating a zombienet-valid dockerfile](#creating-a-zombienet-valid-dockerfile)
+  - [Creating the zombienet manifest](#creating-the-zombienet-manifest)
+  - [Deploying the custom parachain locally](#deploying-the-custom-parachain-locally)
+  - [Updating the parachain image](#updating-the-parachain-image)
+  - [Troubleshooting](#troubleshooting)
+    - [Timeout errors](#timeout-errors)
+    - [Cannot exec into a container in a completed pod](#cannot-exec-into-a-container-in-a-completed-pod)
+
+## Requirements
+- Docker
+- Dockerhub account
+- Minikube
+- Kubectl
+- Node 16
+- Clone and follow the [Zombienet installation and setup](https://github.com/paritytech/zombienet#installation) (PATH setup is optional)
+
+## Initial setup
+
+First, start the minikube cluster.
+
+```bash
+minikube start
+```
+
+Within the Zombienet repo directory, make sure that the tool is deploying the network examples correctly.
+```bash
+cd zombienet
+node dist/cli.js spawn ./examples/0001-small-network.toml
+```
+If the process results in a Timeout error, refer to the [troubleshooting guide](#timeout-errors) taking into consideration the image that zombienet failed to pull.
+
+After some intermediate tables, a successful deployment should print a `Network launched 🚀🚀` table. Note that the polkadotjs endpoints are specified in the `Direct link` rows. Both the relay and parachain should be running at this point.
+![Testnet running on cli](testnet-running.png "Success!")
+
+By pressing `ctr+c` all the processes should stop.
+
+The zombienet manifest that was deployed is a simple relaychain to parachain connection, in which the rococo chain has 2 nodes (alice and bob nodes), while the connected parachain (100) is a template with only one collator:
+```toml
+[relaychain]
+default_image = "docker.io/paritypr/polkadot-debug:master"
+default_command = "polkadot"
+default_args = [ "-lparachain=debug" ]
+
+chain = "rococo-local"
+
+  [[relaychain.nodes]]
+  name = "alice"
+  validator = true
+
+  [[relaychain.nodes]]
+  name = "bob"
+  validator = true
+
+[[parachains]]
+id = 100
+
+  [parachains.collator]
+  name = "collator01"
+  image = "docker.io/paritypr/colander:master"
+  command = "adder-collator"
+```
+The `command = "adder-collator"` specifies the node executable (similar to `./target/release/hashed-parachain`). 
+
+## Creating a zombienet-valid dockerfile 
+The next section will be performed under the parachain repository instead of the zombienet one.
+
+Now that all the tools are functional, the next step is to create a docker image that replaces the parachain template (the file exists in this repository under the name of `collator.Dockerfile`):
+```docker
+FROM paritytech/ci-linux:production
+
+# this dir doesnt exists but zombienet script tries to create a file within
+RUN mkdir /cfg
+
+WORKDIR /var/www
+
+RUN git clone https://github.com/hashed-io/hashed-substrate.git
+
+WORKDIR /var/www/hashed-substrate
+# change to main or develop
+RUN git checkout develop && cargo build --release
+
+EXPOSE 30333 40333 9933 9944 9946
+
+# add binary to usr folder, according to the best practices
+RUN mv /var/www/hashed-substrate/target/release/hashed-parachain /usr/local/bin
+
+# check if executable works in this container
+RUN /usr/local/bin/hashed-parachain --version
+# ENTRYPOINT allows to add parameters/flags via k8 manifests
+ENTRYPOINT [ "hashed-parachain" ]
+```
+
+Build and publish the docker image using the `collator.Dockerfile`, it is recommended to change the tag and name: 
+```bash
+# change abhashed/test-collator to a desired tag/dockerhub-name. Building the image will take a while.
+docker build -t abhashed/test-collator -f collator.Dockerfile .
+# When the process is finished, push the image to the desired dockerhub account.
+docker push abhashed/test-collator
+```
+
+## Creating the zombienet manifest
+Based on the example provided by zombienet, a simple deployment was declared using a compatible `polkadot` image (in this case, v0.9.27), while the parachain nodes will be instanced from the custom docker image (this file exists in this repo under the name `k8-manifests/zombienet-local.toml`).
+```toml
+[relaychain]
+default_image = "docker.io/parity/polkadot:v0.9.27"
+default_command = "polkadot"
+default_args = [ "-lparachain=debug" ]
+
+chain = "rococo-local"
+
+  [[relaychain.nodes]]
+  name = "alice"
+  validator = true
+
+  [[relaychain.nodes]]
+  name = "bob"
+  validator = true
+
+[[parachains]]
+# it can be any number, doesn't need be aligned to the parachain_id declared on the chainspec
+id = 2000 
+cumulus_based = true
+
+  [parachains.collator]
+  name = "collator01"
+  image = "docker.io/abhashed/test-collator"
+  command = "hashed-parachain"
+```
+
+## Deploying the custom parachain locally
+
+It is highly recommended to pull the published parachain image before deploying [using these steps](#timeout-errors), as it is highly probable to get timeout errors due to the image size.
+
+When executing the zombienet command, the only thing that must be specified is the zombienet manifest (in this case, `../hashed-substrate/k8-manifests/zombienet-local.toml`) instead of the example file.
+
+```bash
+node dist/cli.js spawn ../hashed-substrate/k8-manifests/zombienet-local.toml
+```
+
+The process should display an almost identical process from the testing deployment.
+
+## Updating the parachain image
+If there's a change that needs to be locally tested, it is recommended to rebuild and publish the docker image and [download the update on minikube](#timeout-errors).
+
+## Troubleshooting
+
+### Timeout errors
+![Timeout error](timeout-err.png "Timeout error")
+If an image takes too long to download, it will result in a timeout error. It is recommended to pull the required images beforehand.
+```bash
+# Enter to the minikube container through ssh
+minikube ssh
+# Pull the required images, it may take a while
+docker@minikube:~$ docker pull abhashed/test-collator
+# When finished exit the minikube container
+docker@minikube:~$ exit
+# Retry the zombienet deployment process
+node dist/cli.js spawn ./examples/md5.toml
+```
+It is also recommended to check if the image is published by the same name and tag. 
+
+### Cannot exec into a container in a completed pod
+
+![Completed pod error](completed-pod-err.png "Task failed successfully")
+
+This error usually implies there's at least one misconfiguration on the parachain image, the following examples were found:
+- Zombienet tried to create a file in a directory that doesn't exist.
+- The parachain executable (e.g. `./target/release/hashed-parachain`) wasn't found in the provided `command = <executable>` path.
+- Zombienet tried to replace the `para_id` in the chain spec, but it wasn't found.
+- Any other issue that might cause the container to end unexpectedly.
+
+In order to discover the exact issue, the following steps are recommended:
+1. Zombienet produces intermediate pods for each step. Identify the last pod that was instanced successfully before the error logs appeared (e.g. `temp-1`, `temp-2`, etc).
+![Before error](before-completed-pod-err.png "Before the error")
+2. Prepare another terminal with the command for getting the logs for the temporal pod (e.g `kubectl logs -f temp-2 -n <namespace>`), the namespace will be pasted when available on later steps.
+3. In the first terminal, run the zombienet spawn command again, the first table will have the generated namespace, copy the string. 
+![Namespace](zombienet-namespace.png "Namespace")
+4. Paste the string in the namespace option on the second terminal to complete the command (e.g `kubectl logs -f temp-2 -n zombie-8445ff441ef788f160070687bb33f3a0`), and run the command when the first terminal initializes the desired container, in this case, temp-2.
+5. The output should contain an error description. 
+```
+❯ kubectl logs -f temp-2 -n zombie-943cbea55d3294c8968afeca7dcf8bd3
+Defaulted container "temp-2" out of: temp-2, transfer-files-container (init)
+Error: Input("Error parsing spec file: unknown field `para_id` at line 113 column 1")
+copy files has finished
+```
+In this specific case, the para_id field was not recognized by the chain, which means the `#[serde(rename_all = "camelCase")]` line in the `collator/src/chain_spec/mod.rs` was renaming para_id as paraId, causing conflicts in the zombienet internal processes. Removing that line was enough to fix the error.