Persistent buffer between vertex shader executions

[Nacheitor]

I would like to store a variable that was persistent between executions of a vertex shader. I have read a lot of information about SSBO (Shader Storage Buffer Object) but I can't find any working example.

My original code would be this:

precision highp float;

// Attributes
attribute vec3 position;

// Uniforms
uniform mat4 worldViewProjection;
uniform float update;

// Varying
varying vec4 vPosition;

float buffer[5000];

void main() {
	vec4 p = vec4( position, 1. );
	vPosition = p;
	gl_Position = worldViewProjection * p;


	if (update == 0.0) {
		vPosition.z = buffer[gl_VertexID];
	} else {
		buffer[gl_VertexID] = vPosition.z;
	}
}

The problem is that the array is initialized at each execution. How could I solve this?

Thanks in advance.