docs: Add quick-start guide.

docs/quick-start.md

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+# Quick start
+
+Spider is a distributed system for executing user-defined tasks. It is designed to achieve low
+latency, high throughput, and robust fault tolerance.
+
+The guide below briefly describes how to get started with running a task on Spider.
+
+To get started with Spider, you’ll need to:
+
+* Write a task
+* Build the task into a shared library
+* Write a client to manage the task
+* Build the client
+* Set up a Spider cluster
+* Run the client
+
+# Requirements
+
+To run through the guide below, you'll need:
+
+* CMake
+* GCC 10+ or Clang 7+
+* [Docker]
+  * If you're not running as root, ensure `docker` can be run
+    [without superuser privileges][docker-non-root].
+
+# Writing a task
+
+In Spider, a task is C++ function that satisfies the following conditions:
+* It is a non-member function.
+* It takes one or more parameters:
+  * the first parameter must be a `TaskContext`.
+  * all other parameters must have types that conform to the `Serializable` or `Data` interfaces.
+* It returns a value that conforms to the `Serializable` or `Data` interfaces.
+
+> [!NOTE]
+> You don’t immediately need to understand the TaskContext, Serializable, or Data types as we'll
+> explain them in later sections.
+
+For example, the task below computes and returns the sum of two integers.
+
+> [!NOTE]
+> The task is split into a header file and an implementation file so that it can be loaded as a
+> library in the worker, as we'll see in later sections.
+
+_tasks.hpp_
+
+```c++
+#include <spider/client/spider.hpp>
+
+// Task function prototype
+/**
+ * @param context
+ * @param x
+ * @param y
+ * @return The sum of x and y.
+ */
+auto sum(spider::TaskContext& context, int x, int y) -> int;
+
+```
+
+_tasks.cpp_
+
+```c++
+#include "tasks.hpp"
+
+#include <spider/client/spider.hpp>
+
+// Task function implementation
+auto sum(spider::TaskContext& context, int x, int y) -> int {
+		return x + y;
+}
+
+// Register the task with Spider
+SPIDER_REGISTER_TASK(sum);
+
+```
+
+The integer parameters and return value are `Serializable` values.
+The `SPIDER_REGISTER_TASK` macro at the bottom of `tasks.cpp` is how we inform Spider that a
+function should be treated as a task.
+
+# Building the task into a shared library
+
+In order for Spider to run a task, the task needs to be compiled into a shared library that Spider
+can load.
+
+Suppose that spider is added in subdirectory `spider`, we can set up the following `CMakeLists.txt`
+to build the task library:
+
+```cmake
+cmake_minimum_required(VERSION 3.22.1)
+project(spider_example)
+
+# Add spider library
+add_subdirectory(spider)
+
+# Add the task library
+add_library(tasks SHARED tasks.cpp tasks.hpp)
+# Link the spider library to the task library
+target_link_libraries(tasks PRIVATE spider::spider)
+```
+
+To build the shared library, run the following from the root of the project:
+
+```shell
+cmake -S . -B build
+cmake --build build --parallel $(nproc) --target tasks
+```
+
+# Writing a client to manage the task
+
+To make Spider to run a task, we first need to write a client application. Generally, a client:
+
+1. connects to Spider;
+2. submits the task for execution;
+3. waits for its completion—whether it succeeds or fails;
+4. and then handles the result.
+
+For example, the client below runs the `sum` task from the previous section and verifies its result.
+
+_client.cpp_
+
+```c++
+#include <iostream>
+#include <string>
+
+#include <spider/client/spider.hpp>
+
+#include "tasks.hpp"
+
+auto main(int argc, char const* argv[]) -> int {
+    // Parse the storage backend URL from the command line arguments
+    if (argc < 2) {
+        std::cerr << "Usage: ./client <storage-backend-url>" << '\n';
+        return 1;
+    }
+    std::string storage_url{argv[1]};
+    if (storage_url.empty()) {
+        std::cerr << "storage-backend-url cannot be empty." << '\n';
+    }
+
+    // Create a driver that connects to the Spider cluster
+    spider::Driver driver{storage_url};
+
+    // Submit the task for execution
+    spider::Job<int> job = driver.start(sum, 2, 3);
+
+    // Wait for the job to complete
+    job.wait_complete();
+
+    // Handle the job's success/failure
+    auto job_status = job.get_status();
+    switch (job_status) {
+        case JobStatus::Succeeded: {
+            auto result = job_status.get_result();
+            if (result == job_status.get_result()) {
+                    return 0;
+                } else {
+                    std::cerr << "`sum` returned unexpected result. Expected: 5. Actual: " << result
+                            << '\n'.
+                return 1;
+            }
+        }
+        case JobStatus::Failed:
+            std::pair<std::string, std::string> error_and_fn_name = job.get_error();
+            std::cerr << "Job failed in function " << error_and_fn_name.second << " - "
+                   << error_and_fn_name.first << '\n';
+            return 1;
+        default:
+            std::cerr << "Job is in unexpected state - " << job_status << '\n';
+            return 1;
+    }
+}
+
+```
+
+When we submit a task to Spider, Spider returns a `Job` , which represents a scheduled, running, or
+completed task (or `TaskGraph`) in a Spider cluster.
+
+> [!NOTE]
+> `Job`s and `TaskGraph`s will be explained in another guide.
+
+# Building the client
+
+The client can be compiled like any normal C++ application except that we need to link it to the
+Spider client library.
+
+Again, suppose that spider is added in subdirectory `spider`, we can set up the following
+`CMakeLists.txt` to build the client program:
+
+```cmake
+cmake_minimum_required(VERSION 3.22.1)
+project(spider_example)
+
+# Add spider library
+add_subdirectory(spider)
+
+# Add the client
+add_executable(client client.cpp)
+# Link the spider library to the client
+target_link_libraries(client PRIVATE spider::spider)
+```
+
+To build the client executable, run the following from the root of the spider project:
+
+```shell
+cmake -S . -B build
+cmake --build build --parallel $(nproc) --target client
+```
+
+# Setting up a Spider cluster
+
+Before we can run the client, we need to start a Spider cluster. The simplest Spider cluster
+consists of:
+
+* a storage backend;
+* a scheduler instance;
+* and a worker instance.
+
+## Setting up a storage backend
+
+Spider currently supports using MySQL or MariaDB as a storage backend. In this guide, we'll start
+MariaDB in a Docker container:
+
+```shell
+docker run \
+        --detach \
+        --rm \
+        --name spider-storage \
+        --env MARIADB_USER=spider \
+        --env MARIADB_PASSWORD=password \
+        --env MARIADB_DATABASE=spider-storage \
+        --env MARIADB_ALLOW_EMPTY_ROOT_PASSWORD=true \
+        --publish 3306:3306 mariadb:latest
+```
+
+> [!WARNING]
+> When the container above is stopped, the database will be deleted. In production, you should set
+> up a database instance with some form of data persistence.
+
+Alternatively, if you have an existing MySQL/MariaDB instance, you can use that as well. Simply
+create a database and authorize a user to access it.
+
+## Setting up the scheduler
+
+To build the scheduler, run the following from the root of the project:
+
+```shell
+cmake -S . -B build
+cmake --build build --parallel $(nproc) --target spider_scheduler
+```
+
+To start the scheduler, run:
+
+```shell
+build/src/spider/spider_schdeuler \
+        --storage_url \
+        "jdbc:mariadb://localhost:3306/spider-storage?user=spider&password=password" \
+        --port 6000
+```
+
+NOTE:
+
+* If you used a different set of arguments to set up the storage backend, ensure you update the
+  `storage_url` argument ihn the command.
+* If the scheduler fails to bind to port `6000`, change the port in the command and try again.
+
+## Setting up a worker
+
+To build the worker, run the following from the root of the project:
+
+```shell
+cmake -S . -B build
+cmake --build build --parallel $(nproc) --target spider_worker
+```
+
+To start a worker, run:
+
+```shell
+build/src/spider/spider_worker \
+        --storage_url \
+        "jdbc:mariadb://localhost:3306/spider-storage?user=spider&password=password" \
+        --port 6000
+```
+
+NOTE:
+
+If you used a different set of arguments to set up the storage backend, ensure you update the
+`storage_url` argument in the command.
+
+> [!TIP]
+> You can start multiple workers to increase the number of concurrent tasks that can be run on the
+> cluster.
+
+# Running the client
+
+To run the client:
+
+```shell
+./client "jdbc:mariadb://localhost:3306/spider-storage?user=spider&password=password"
+```
+
+NOTE:
+
+If you used a different set of arguments to set up the storage backend, ensure you update the
+storage backend URL in the command.
+
+# Next steps
+
+In future guides, we'll explain how to write more complex tasks as well as how to leverage Spider's
+support for fault tolerance.
+
+[Docker]: https://docs.docker.com/engine/install/
+[docker-non-root]: https://docs.docker.com/engine/install/linux-postinstall/#manage-docker-as-a-non-root-user