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Running a Prolog Script

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Introduction

Prolog is intended primarily as a declarative programming language: the program logic is expressed in terms of relations, represented as facts and rules. A computation is initiated by running a query over these relations. Prolog is well-suited for specific tasks that benefit from rule-based logical queries such as searching databases, voice control systems, and filling templates.

This tutorial is a quick guide on how to run a hello world script on Bacalhau.

Prerequisites

To get started, you need to install the Bacalhau client, see more information here

1. Running Locally​

To get started, install swipl

sudo add-apt-repository ppa:swi-prolog/stable
sudo apt-get update
sudo apt-get install swi-prolog

Create a file called helloworld.pl. The following script prints ‘Hello World’ to the stdout:

# helloworld.pl
hello_world :- write('Hello World'), nl,
               halt.

Running the script to print out the output:

swipl -q -s helloworld.pl -g hello_world

After the script has run successfully locally, we can now run it on Bacalhau.

Before running it on Bacalhau we need to upload it to IPFS.

Using the IPFS cli:

wget https://dist.ipfs.io/go-ipfs/v0.4.2/go-ipfs_v0.4.2_linux-amd64.tar.gz
tar xvfz go-ipfs_v0.4.2_linux-amd64.tar.gz
mv go-ipfs/ipfs /usr/local/bin/ipfs
ipfs init
ipfs cat /ipfs/QmYwAPJzv5CZsnA625s3Xf2nemtYgPpHdWEz79ojWnPbdG/readme
ipfs config Addresses.Gateway /ip4/127.0.0.1/tcp/8082
ipfs config Addresses.API /ip4/127.0.0.1/tcp/5002
nohup ipfs daemon > startup.log &

Run the command below to check if our script has been uploaded.

ipfs add helloworld.pl

This command outputs the CID. Copy the CID of the file, which in our case is QmYq9ipYf3vsj7iLv5C67BXZcpLHxZbvFAJbtj7aKN5qii

Since the data uploaded to IPFS isn’t pinned, we will need to do that manually. Check this information on how to pin your data We recommend using NFT.Storage.

2. Running a Bacalhau Job

We will mount the script to the container using the -i flag: -i: ipfs://< CID >:/< name-of-the-script >.

To submit a job, run the following Bacalhau command:

export JOB_ID=$(bacalhau docker run \
    -i ipfs://QmYq9ipYf3vsj7iLv5C67BXZcpLHxZbvFAJbtj7aKN5qii:/helloworld.pl \
    --wait \
    --id-only \
    swipl \
    -- swipl -q -s helloworld.pl -g hello_world)

Structure of the Command

  1. -i ipfs://QmYq9ipYf3vsj7iLv5C67BXZcpLHxZbvFAJbtj7aKN5qii:/helloworld.pl : Sets the input data for the container.
  2. mYq9ipYf3vsj7iLv5C67BXZcpLHxZbvFAJbtj7aKN5qii is our CID which points to the helloworld.pl file on the IPFS network. This file will be accessible within the container.
  3. -- swipl -q -s helloworld.pl -g hello_world: instructs SWI-Prolog to load the program from the helloworld.pl file and execute the hello_world function in quiet mode:
    1. -q: running in quiet mode
    2. -s: load file as a script. In this case we want to run the helloworld.pl script
    3. -g: is the name of the function you want to execute. In this case its hello_world

When a job is submitted, Bacalhau prints out the related job_id. We store that in an environment variable so that we can reuse it later on:

3. Checking the State of your Jobs

Job status: You can check the status of the job using bacalhau job list.

bacalhau job list --id-filter ${JOB_ID} --wide

When it says Published or Completed, that means the job is done, and we can get the results.

Job information: You can find out more information about your job by using bacalhau job describe.

bacalhau job describe ${JOB_ID}

Job download: You can download your job results directly by using bacalhau job get. Alternatively, you can choose to create a directory to store your results. In the command below, we created a directory (results) and downloaded our job output to be stored in that directory.

rm -rf results && mkdir -p results
bacalhau job get $JOB_ID --output-dir results

4. Viewing your Job Output

To view the file, run the following command:

cat results/stdout

Support

If you have questions or need support or guidance, please reach out to the Bacalhau team via Slack (#general channel).