1. A system for content delivery by simulating user web page visits, the system comprising: using a content server, that comprises one or more first processors and one or more first memories, to perform first actions including: receiving, from a client device, a request that includes one or more constraints and is for data corresponding to user sessions characterizing web page visits that satisfy at least one of the one or more constraints; and sending the request to a simulation server; using the simulation server, that comprises one or more second processors and one or more second memories, to perform second actions including: transmitting, to a visit database, a query for session records that include a timestamp within a predefined time period; receiving, as a result of the query and from the visit database, a set of session records, each session record of the set of session records comprising a set of record attributes, the set of record attributes identifying a user ID, a URL, and a first timestamp, wherein the first timestamp indicates a time of a visit to the URL by a user device associated with the user ID; generating, for each session record of the set of session records, a simulated HTTP call of a set of simulated HTTP calls for the session record, wherein the simulated HTTP call includes one or more properties, wherein a first property of the one or more properties corresponds to a record attribute of the set of record attributes included in the session record, and wherein a second property of the one or more properties includes a second timestamp that is different than the first timestamp; sending, to a visit server, the set of simulated HTTP calls; using the visit server, that comprises one or more third processors and one or more third memories, to perform third actions including: receiving the set of simulated HTTP calls; for each simulated HTTP call of the set of simulated HTTP calls: identifying, using the first property of the simulated HTTP call and from amongst the set of session records, a session record that corresponds to the simulated HTTP call, as an identified session record of a set of identified session records; and updating the identified session record to include one or more updated record attributes that represent a simulated session associated with the second timestamp and corresponding to the simulated HTTP call to produce an updated session record of a set of updated session records; sending the set of updated session records to the content server; further using the content server to perform fourth actions including: identifying a subset of the set of updated session records, wherein, for each updated session record in the subset, a constraint-processing condition is satisfied based on the set of record attributes in the updated session record; generating a response communication that includes, for each updated session record in the subset, data from the updated session record; and transmitting, to the client device, the response communication.